In Caleb's interview at the FLOM4 web site he says (these aren't exact quotes, you'll have to read the article--if you care ):
"Rachel's giving up the million dollars was a nice gesture." (Gesture? I'll say. Rather stupid and not all that well thought out, too.)
"I guess I forgot to tell her that I'm a major commitment-phobe." (Yeah, that might have been an important piece of information to share.)
"We've realized that we're better off being just friends." (No offense, but if you forgot to share that information with me and I lost a million dollars over it, I wouldn't exactly consider you major friend material.)
Oh, and P.S., he figured the odds were pretty great that he didn't have a million dollar mystery check, he had watched both FLOM1 and 2 and he assumed that she was playing for money, and he was expecting that if she chose him and he chose her over the money they'd get to split the million dollars (or however much she was playing for).
Turns out good old Caleb was more slimey than we ever thought Rachel was.