base_sell_price() in src/shandalar/replace_functions() controls sale price in the deckbuilder (player selling cards in a town). There's a somewhat less up-to-date but more legible-to-nonprogrammers version here. Examining card_rating_impl() in src/shandalar/ai.cpp might be a good start for ideas, and the AI Base Value column in manalink.csv should definitely be included.

I'm not yet certain where the price for buying cards from a town, or from a Nomad's Bazaar lair, are.