I kinda like this. The current "best practice" is to only upgrade to full levels, without blocking more BCX than necessary. And rent out lower-level cards (usually at level 1, thus why so many level 1s on the market, besides the actual demand which is for level 1s) until there are enough to level up.
What you propose offers an alternative incentive for combining cards without reaching the next full level, while being on the same page with what the proposal wants to accomplish: to determine people to combine their cards.