You're talking about something that is quite a bit different than converting to a fixed gear. What you're talking about is attempting to make something into something it isn't. Converting to a fixed gear isn't cheap really. Maintaining a fixed gear tends to be cheap, but building one that will last is probably going to cost you a couple hundred bucks at least. Find someplace you can save money or something you can sell and build a decent bike, 'cuz what you're talking about can get you killed, as can riding a bike with a freewheel brakeless.