First of all, prices are determined by server. Not sure about the others, but Teva has seen prices for everything skyrocket. Apparently we have too much money there. Anyway, then your price is determined by class and stars. I have seen a low level Class C with 4 stars going for 500Silver and a Dungeon Cornelius (Dragon's Tomb Boss) sell for 20Gold. Ultimately, you sell for what you think you can get for it and buy for what it is worth to you.
In my case, I will be buying some low level cards when I can find them since my mains were high level before the card system started, and thus might overpay to get something I need.
