He typically loses things that are of value to heroes but it takes a lot more to outsmart him and make him give up what *he* values. He's not like the Shocker where he's just some dude in a suit looking to get paid, and he doesn't give a shit about world domination - but he does care about causing chaos and upsetting order.
Case in point: the Parker marriage. He fixed everything for Peter - his identity wasn't the only thing he got back. He got his aunt (his only family), he got his spot on the Avengers (although really he probably never lost that, he might have given it up to protect MJ and gone into hiding), he got himself back, ie he stopped being the angry, black-suited rage-driven guy he briefly was.
Mephisto on the other hand got the knowledge that Peter had given up the thing he loved and only Peter and Mephisto knew what he'd done. He'd taken the squeakiest of clean guys and made him dirty on a fundamental level. It's a shame they never played to that - Pete's angst does seem to sell after all - but with the backlash you can understand why they moved on.
But then the flipside of that deal is that the way Mephisto had it happen wasn't just a click of his fingers, it played out "naturally" so that nobody would be any the wiser as to the deal; so Stark and Richards and Strange did the legwork, and MJ and Peter both remembered the marriage, and Aunt May recovers "on her own", but Peter and MJ were never married so it becomes a paradox - did Mephisto really make a deal with *this* Peter, who was never married, or does he just remember a different universe in which he was married alongside the one in which he wasn't?
You're left not knowing how much Mephisto really gains, but that's the essence of the devil-makes-an-offer stories: if he's not asking outright for your soul, you've got a chance to play him at his own game and win, if you're good enough - and as long as you've understood the game.