Hearthpwn

Is there a way to make this deck cheaper? Like which cards would you exchange?

Wow, I didn't realize how many legendary cards there were in this deck before your mentioned it!

I have all of these now as I've been playing Quest Priest since the release of the card, but as someone who didn't have many cards and added all of them one by one, I can give some of my notes on those legendary cards to shine light on why they are good and their purpose, and what I had swapped them with when I didn't have enough for them yet.

If you want to play Quest Priest, your main card is "Seek Guidance" of course. Here is for the rest of them:

Must haves:

• Taelan Fordring: used to draw one of your 7 cost card minion, allowing you to ensure that you can continue your quest steps after you complete 5-6, so this card is good to have for that reason. I had this card from the start, but when I tried running without it, I found myself having a hard time getting my 7 cost card. I only had 30 cards when I had this issue, imagine with 40 cards in your hand, only 2 chances of getting a 7 cost card? Even your dredge from Illuminate isn't enough to get it most of the time. This gives you another opportunity of getting it, so for that reason, I suggest you invest your dust to get this card.
• Blackwater Behemot: used to heal yourself and clear some of the enemy's board, because the colossal minion it spawns makes a random enemy minion attack Behemot, so it will give you a +8HP, while probably killing that minion. Plus, if that minion doesn't kill Behemot, on their turn, they might have to attack Behemot again to kill it, giving you even more HP back. Turn 7-8-9 is usually weak turns for Quest Priest, because you might die from not having enough health remaining at this point before the end of your quest, so this is used for that purpose. Even though Prince now makes you start with more health, I still think Behemot is good to have, especially because Priest doesn't have many 7 mana cost card that are worth swapping it for, so I would also suggest getting this card too.

Other cards:

• Xyrella: board clear is very good to have with quest priest, because you don't have a lot of board presence. Since this deck has desperate prayer, this gives you an opportunity to heal yourself AND clear the enemy minions. So, without it, you don't get the most use of your heals, but you can play without it if you don't have enough for it. If you don't play with it, I also recommend swapping "Desperate Prayer" for "Gift of the Naaru", it gives a bit less heal, but it will allow to have access to your other cards faster. As for what to swap Xyrella for, I'd go for "Spammy Arcanist" as it still allows you to clear the board in another way, plus it adds another 5-cost mana card, which this deck would need more of.
• Mutanus: used to counter your opponent's quest card if they have one also, as it gives you a chance to eat them out of their hand before they get a chance to play it and activate its effect, it's so satisfying when it happens! If they don't have a quest, it can still be a good opportunity to maybe deny one of their big minions. There is nothing equivalent you can swap it for, but since you have x2 "Drown" in this deck, you still have other opportunities to remove a dangerous minion from the board. You will still need a 7 cost card to replace it because of your quest, so I suggest replacing it for "Slithering Deathscale", it's similar to Lightmaw Netherdrake, although it's less reliable and costs more, but I still find it viable as you need to keep a 7 cost card to replace it. Another similar card to these is "Baron Geddon", it's more reliable than Slithering, but it's another legendary card, so use it instead only if you have it, otherwise don't invest in it and simply get Mutanus later on.
• Blademaster Okani: this is another card that can potentially deny a good spell or minion or make your opponent burn one of his card to prevent the effect of this card, so this can make you gain a turn advantage on your opponent and win more time for your quest. This is the only card of it's kind, so there is no other way for a Priest to prevent a spell to be played, but if you don't have this card, another card that can help with potentially winning time would be "Disarming Elemental" by setting his next card drawn to cost 6 mana. This is especially good when you play it early game, as he won't be able to play it right away.
• Najak Hexxen: very good if they have minions that you might not have the mana or card to clear yet, this can give you a turn or two of using his minion against him until you are able to remove it from the board or kill it by yourself. I ran this deck without it for the longest time before I had enough dust for it. I didn't have an issue playing this deck without it, but I still feel the difference with it now because it's a good card. Although, since this deck has x2 "Drown" already, it can be viable without Najak in it, as you have other opportunities to remove an enemy minion from the board. An alternative option could be "Shadow Hunter Vol'Jin", as it can help temporarily remove an enemy minion from the board, as they will have to play it again, but it's another legendary card, so swap it for it only if you have it. Otherwise, any other 2 to 4 mana cost minion can do. 
• Xyrella, the Devout: this card will give you the opportunity to heal yourself again with Lightshower Elemental or attack your enemy again with Undying Disciple, or even both! This is the third card I invested into, because reusing your cards a second time by triggering their deathrattle rattle is really good, plus after you will be able to heal +5HP every two turns (or attack 5). There is nothing you can swap it for that is similar, so if you do without it, you can simply add any lower cost card as you don't necessarily need another 8 cost card, because at this point you should already have Whirlpool in your hand for your quest after using Thrive or Spirit Guide. 

Swapable cards (to reduce the amount of spells in your deck)

• Illuminate: can be replaced by "Tuskarrrr Trawler" as both enables you to better control the cards that you will pick next to ensure that you have all the cards that you will need to continue your quest. I think this deck has enough spell as is, so this could be a nice swap, plus it would give you a little more board presence early on.
• Shadow Word: Ruin: could be swapped by "Spammy Arcanist" as they both cost the same mana and have good clear potential. At turn 5, there might not be a lot of big attack minions on the board yet, so spammy arcanist could have a better chance at clearing more minions. 

I hope these tips helped you make your choice as to which card you find would be worth to invest into or if there is a way for you to make it cheaper while it still works for you.

Wow thank you so much for this thorough answer!

My pleasure! If you have any other questions, feel free to ask! I don't run this exact build, but I've been playing many variations of Quest Priest.

at least is not some of those "1000% Winrate Legend OFF META BREAK SUPER DUPER FCER ULTRA FINAL DECK" on title.

I was playing warlock vrs this deck 3 seperate time in diamond..it got dumpstered every time, 

Why would you want to play quest with more cards? 
When I play quest, I want to go through my deck as fast as possible. The longer the game takes, the more change you have of losing.

It's because you get 10 extra hp for free at the start of the turn. And the meta is so fast that even that much hp is a huge help.

Because you have a greater chance at getting those pesky 5 and 6 cost cards AND 10 more HP helps you stay alive longer!

how to interrupt this? before last expansion, you could steal the purified shard and block your enemy from playing it next turn, but now?

celestial druid, lock with altar of fire, or just eat the quest reward with mutanus

Immolate with curselock is awesome against this.

so need to play cancer with cancer? 

And "just" eat the reward is not that good when Priest hands are usually full due to too much draw and discover

Way too many spells in this deck, and that's bad when you use Thrive to find win condition.

illuminate, thrive, spirit guide, Xrella, Entrapped Sorceress............. 

Actually thrive is AMAZING at getting the 10 cost OTK.  Rarely does it not work.