Someone might have already mentioned this downthread--I'm just now logging on and starting to read.
I would hazard a guess that the money is coming from a book and/or "movie of the week" deal. Back when they were holding fund-raisers news conferences pleading for their daughter's return, Brenda was heard asking someone about who would be the best person to represent them in such a deal. This would have been before the time they found Danielle.
I'm sure they found the public donations helpful, but I don't think those would pay for the BMW and the house--even with the insurance money tossed in, unless she was insured for far, far more than we imagine.