Abduction of an American Playgirl (1975)

Two horny guys with no money spot a pretty blonde walking down the street. They kidnap her and take her to a remote cabin owned by the sister of one of them, planning to have sex with her for a few days, then turn her loose. They discover that she is actually the daughter of very wealthy parents, and decide that after they're done with her, they'll hold her for ransom. It turns out that the girl is not quite the spoiled heiress they think she is, and soon they learn that they're in for a lot more than they counted on.