Well, the gun may be worth about $350 to $400. You can get a brand new G-2 Contender with barrel of choice for around $450 + or -. The ammo may be worth nearly as much, to the right person, if original Remington 158 grain stock. They haven't made that stuff in years and it does have collector value. You can buy "custom" Max ammo from places like Reeds Ammuniton and Research for about $12 for a box of 20, so the face value for 8 boxes of 50 would be around $240. All in all I'd say it's a bit high, for a shooter (gun and ammo) but we all start high so there's room to come down. Offer $550 and go from there.
Dang, I gotta learn to type quicker ;-)