what they should do if they wanna make us have to craft em is if you complete the dungeon in bronze you dont get a key if you complete silver you get +1 key back and if you complete gold you get +2 keys back.
Now obviously they probably wouldnt wanna do that for each time you complete silver or gold but maybe atleast the first silver and first gold you complete each week for whoever used the key.
so you can craft +2 a week (maybe allow bonus crafts) but you also will get +1 for silver and +2 for gold
So a total of 5 mutation keys a week (maybe 1 or 2 more if they allow bonus crafts and you get lucky twice)