Only way to fly "cheap" and legally on a small plane with a private pilot would be if the pilot is a genuine friend of yours .....
That's overstating the case actually. For one thing, the FAA is hardly in a position to determine what constitutes a "genuine friend" (actually I wish I knew myself). The term which crops up is "shared purpose".
So... if my best buddy from school who I've known all my life says, I want to go to Vegas, could you fly me there, and he pays half the costs, I'm on thin ice. But if I'm preparing my flight and a total stranger walks up to me and says, "Hey I see you're planning a flight to Vegas, any chance I can come along with you and share the costs" that is most likely OK (though maybe not prudent in the event that there's an accident and he or his family sues).
Of course there's a huge grey area here and realistically, as long as there's no accident and nobody complains, the chances of the FAA taking any action are extremely small. But them's the rules.