For many of these cards, we'll have an exact match by XP in the market, but sometimes the market does not have a card with the same XP so it's hard to judge value. In this case, I've looked at the market and found the value of the 'next lowest XP' in the market. In this case, we see the second card says, '120 card XP, 100 market XP' and if you click that you'll go straight to the market to see the value of cards at that XP. The idea here is to get the value of the card that is closest to yours.
This is very bad idea :) It should be done my looking at value of single card and multiply by BCX of given car.
VS
I'm not sold on BCX, that honestly seems to confuse things more than explain them so I'm thinking about getting the raw 'source cards count' to show how many cards were combined to reach the current level. Then I can show raw cost per card.
The issue with this and the approach you mention is that the 'cost per [card/XP/BCX]' will be different at every XP level, so then value which do I use for a specific card?
It's a 'rough guess' no matter what if you don't have an exact match so I'm curious what everyone thinks would be most accurate.