With your mechanical skills you could very easliy go the craigslist or used route. You can get a pretty good bike for less than $100 sometimes really nice bikes for less than$300 and if you want to spend some money to get it really nice. If you just want a functional bike just get a 90's steel frame bike (these are dirt cheap sometimes free) and put some different tires on it probably a new chain and ride it all over the place. Get a 2nd set of wheels for it and put some fat mountain bike tires on is and ride the trails.

If you are willing to tell us where you are located we could point out some bikes on your area craigslist that would be good to check out. Also how tall are you? Garage sales are places to look and also sometimes pawn shops can have deals. If you lived here and were the right height there is a Gary Fisher for $150 that would be a good deal at a Pawn shop near my house. If you are on facebook you could try their looking in their marketplace ads. If you want you can post some Craigslist bikes you are interested in and we can let you know what we think.