While the Gardiners seem comfortably off, I'm sure they're not richer than the Bennets, or even as rich. If he had really bought off Wickham, Mr. Gardiner would be destabilizing his own children's futures, if not actually taking food out of their mouths. I always thought it would be easier for Mr. Bennet to repay his BIL than for Mr. Gardiner to find the money in the first place. I think that here Mr. B. is not just referring to the financial debt to his BIL (heavy though it might be), but to the *psychological* debt too, which can never really be repaid.