[Hectic]

My first JG m4 was like 600 bucks new, plastic body and all. Now you can get them for 200 lol.
That being said, used stuff was once as much as new. These days in reality (due to availibility) stuff should be 20-50% off of retail based on milage and condition, plus 20-50% of the cost of upgrades, again depending on milage.
If I buy a vfc and swap the polycarb piston for a lonex one and list that as an upgrade it is, and price should reflect that, again as an example if the gun was 500 new and the piston 40 the gun should resell for 270-430 depending on condition, example, "bought gun, swapped piston, played 1 game, gun too heavy" should fetch closer to the 430 mark. "Bought gun,; custom paint, played with it for t seasons, wanna try something new" should be on the low end.
as Brian said, don't like the price, don't buy, prices will have to drop.