I just picked up a Trek 7100 that was refurbished (new shifters, new tires, new grips, paint touched up) for $150. I love the bike, and it can definitely keep up with the 15 mph+ rides.
I haven't ridden a bike in years (though I'm in fairly decent shape) and was able to keep an 18 mph pace on flat road and 29.6 downhill.
Unless it's beat to hell I'd say $70 is a very good deal.