I think we're at the point where there is a premium for top condition M15s... they were mostly service guns, and the condition of the typical M15 on the market reflects that. A 'like new' or unfired gun will command a significantly higher price. The average gun may be a $550-650 gun, but I don't think $700+ is too much for a high condition example.
I recently sold a 15-3 for $700. It was a factory reblue, back when they were doing it right, and it looked like a brand new gun. The dealer I consigned it through didn't think it would bring that, but it did, and quickly.