Renting 15k or even 40k is worthed on avarage,with 1 day renting you can do 2 days Quests with them, meaning 4 or 6 extra chests, now a lot more importand for next seasson since bronce reward has been reduced, however renting on the last day is not worthed at all unless you want to past all the day waiting for a lucky cheap card and you are lucky enough to get it before anyone else, on avarage you will get better deals by renting for 2, 3 or even 4 days before the seasson is over, on the last day the renting prices can become almost as higher as the buy prices O.O