I don't think a used Grace frame would be worth that much. One of the problems I've seen with Grace frames is that the rails tend to bow in the middle, especially on the larger frames. Hinterberg frames are very good. They are no longer made, but the company still sells used ones:
http://www.hinterberg.com/stretchframe.aspx
Also, it really is best to buy a frame and machine together so you can be sure they work together. Not every frame can handle every machine. If the Grace frame is working with a domestic sewing machine, then you can be pretty sure it can only handle a domestic sewing machine.
Edit: I just saw the photo you posted. I'm really not sure about that frame, but it seems to me the carriage is the size that can handle only a domestic sewing machine. Might be able to purchase an upgraded carriage that can handle a midarm, but they can be pricey. If the frame is old, it might be hard to find a larger carriage for it. You might want to find out how large a space you can quilt. The limitation is usually that, especially with a large quilt, after rolling you end up with only 3 or 4" of width available to quilt in.