IME, you can find a fixed gear rear wheel for 60, but can convert just about any rear hub to a fixed gear or at least single speed for next to nothing. but as previously mentioned, you'll either have to have a frame that's old enough to have been manufactured with forward facing horizontal dropouts meant for a derailleur equipped bike, or a track frame with rearward facing horizontal dropouts or get an expensive ENO eccentric rear hub or have a frame with a BB30 or PF30 bottom bracket and buy an equally expensive eccentric bottom bracket.