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.