My brother recently got a lightly-used 2012 Specialized P.20 AM off craigslist in HI for $275.

He said the dood ate it first time on some dirt jumps and didn't want to ride it anymore.

I looked at that bike and for $100 it might be worth it to start out with. The person has a bike stand so maybe they have maintained it too. Check the crank for play, frame for hairline cracks etc.