The cost of an expansion has more to do with what your expectations and requirements are. It also has to do with your ability to wait.
If you want to play every deck and have every card its going to cost a lot.
If you want to play a few of the top decks and have fun the cost is a lot less.
Its possible to save 10k dust between expansions. It takes time and dedication, but its possible. You can also spend money if you dont have the time. Nothing in life is free. If you open 100 packs its possible to play a few top decks and have fun.
Next if you play on a budget you have to be wise in your crafting. You cant rush right after release and craft what may be good. You have to wait for the meta to settle and let others waste dust testing the cards. Then a few weeks into the meta you craft the best.
Lastly keeping gold cards is nice if thats what you want. But you are inflating the cost of the game by keeping them. Gold cards have no advantage during play. They just look pretty. If you want to keep cost s down dust gold cards you have the max normal versions of.