There are a lot of cheap motherboards in the $60 range, and a lot in the $100 range. Going cheap, however, may be more expensive once you factor in the cost of other components.
For instance, DDR3 2GB ram is $60/piece. DDR 4GB ram is $250 (prices crucial.com). So if you get a cheap MB with just two ram slots, you are pretty much limited to 4GB of ram, at least at economical prices. 4GB might be enough for anybody (today) but for how long? Ram usage seems to go up a lot quicker than CPU usage.