Craigslist is your friend. We got a Giant for youngest DS for $100. Avoid big box store bikes like the plague. They are guaranteed to be junk not because the frames are junk but because the components - the brakes and derailers, etc. are junk. You want to get a bike that came from a bike shop that has a shimano derailer. Trek, Giant, Specialized, Cannondale are names to look for.
One other big thing to look at before you buy a bike for your son. Make sure it's going to fit. Just because it's a 26" bike doesn't mean it's going to fit. Some have 26" tires and a 19" frame and some have 26" tires and a 14" frame. Measure his inseam and measure the center height of the bar of the bike before buying. You want at least 2" clearance between his crotch and the bar.
