This was kind of a weird one I thought of, but would it be feasible to add in a popularity section for carts to indicate that a cart may be common, but popularity would demand more cash.  Contra and Bubble Bobble come to mind here.

Or would this be too hard to be credible?

I think this was discussed here a couple of months back, perhaps in the rarity discussion thread.

Dain's rarity numbers are really scarcity numbers. They take into account current availability for two reasons:

1. No real production quantities exist, so a full blown relational rarity guide based on how many would exist if none were ever lost or destroyed would be impossible.

2. Current availability based on real data compiled through eBay and general experience hunting, by its nature takes into account available quantity, factored with popularity (i.e., how many people keep it once they buy it, for whatever reasons...or at least that they keep it for longer periods - less turnover = less availability).