Ruby claims she can help Sam save Dean, and maybe she can, but it's bait. There's no sense that she wants to save Dean. She doesn't care. She wants something from Sam and she's using Dean to get it.

Bella's attitude is "everything has its price". On the one hand, Dean's already made his deal and now he has to pay up. On the other, I think Bella would (if she knew about the deal) take the attitude that there's got to be a way he can buy himself out of it. And I think she's well placed to have a clue what that way might be...if their relationship progresses to the point where she gives a damn.