Actually the Jamis that Muirenn linked to is pretty good.

I got my beater bike used from a bike rental company on the Upper West Side. I paid $100 for a 2010 Giant Cypress in good condition. The frame is steel, the seat is nice and comfy and the ride is smooth. A new one is about $350 but since this was a beater to put on a trainer, I wanted to spend the least amount possible. The Giant Cyress is actually considered a "Comfort" bike - great for beginner riders - it feels like I'm riding a couch!

http://www.giant-bicycles.com/en-us/....w/3919/36573/