I would say no, not unless the drained levels have become permanent.
Say a 10th lvl human PC gets hit by a vampire and drained 2 levels.
He's still a 10th-level character, he just has two "negative levels" stacked on him.
If he fails both saves after the 24 hours are up, then he's an 8th-level character with no "negative levels", and at that point you would pay the XP cost for an 8th-level character.
So no, you can't massively level-drain a target, hit it with Monstrous Thrall for a decreased cost, then Greater Restoration it back to full strength.