Yeah, some more context would be appreciated. But from what I can infer from your post, here goes:
How can I say how well you did on a level?
You'd have to make preserving blocks challenging. This also depends on whether preserving blocks is the point of the game or an optional challenge to get more money. In that case, the number of preserved blocks wouldn't tell you how well you performed on a level, but how well you performed on the optional challenge.
You could also make it a tradeoff (which it sounds like it is already). Not preserving blocks would give the player some kind of advantage but preserving them would mean the player could buy more upgrades or whatever the "shop" sells which would result in a different kind of advantage. The first option would be instant gratification while the second one makes the player wait a bit for their reward. Of course the rewards would have to be different as well for this to work.
My game 444 has something like this. You get experience points (the game's currency of sorts) for killing enemies, but the enemies (and thus the points) are finite, so the player has to spend them strategically. In my case the choice is between stat upgrades and new weapons.