I don't think there are any. I think you have to go pretty high end with any of the big guys to get an American frame if they make any at all. Some of the small guys are still American made but I doubt you can find a frame from any of them for $400 let a lone a whole bike
http://oldglorymtb.com/mountain-bikes-made-in-america/
don't know how up to date that is

The gravity still shows up as $400 for me.