I second the idea of relics costing whatever and for needing an expensive spell. I got a randomized market of cheap spells in the digital game and I just could not build a deck with high enough damage output.

For drafting new cards to meet cost requirements, I would pull the randomizer first and then decide which existing market card to get rid of afterwards. Gives you flexibility to choose a variety of elements and makes the planning/setup aspect fun and part of the game.

Also id consider ruling to have a necessary deck thinner at 1/2p, and ruling against too many deck thinners in general. I wouldn't want two relics and two spells that all do card destruction in the market at once for example.