Unless you have some specific requirements, buying an entire bike is the way to go. At least around here, early Treks and Schwinn LeTour type bikes are very reasonable. Most times if you see a frame, it's been stripped to sell off piece by piece by a flipper so you may end up paying just as much for a frame as an entire bike. It's fun to build a bike frame up - but depending on your situation, the most expensive too.