If you don't play arena ever at all: start an arena run (150 gold), you don't even have to finish the draft. You'll get a pack as a reward + free ticket with which you can start another arena run to insta-retire and get another pack. 2 packs for 150 gold. 50 gold profit.
I believe you do have to finish the draft. I am not sure if you have to play any matches though. I know that on a draft I didn't complete when I got the prizes the pack didn't actually show up for me to open nor did I get the gold. So I will not be trying that again.