Most Nylon 66s I see for sale are in the $200-$300 range depending on their condition. I saw a Nylon like that one on EE not long ago on for $300, if memory serves me, though yours looks to be in better shape esthetically. Assuming the rifle is good shape functionally and the fact it is stainless I would say that 300ish is not an unreasonable price. Even $350-400 if its in exceptional shape. Nylons have a great reputation and that one is a little more unique then the standard blued ones.
I'm no pro this just what I've observed in spending to much time on sites like Ellwood Epps and the EE.