20th Anniversary!

I have long wanted this coin , although I dont collect all of the Silver Eagle's (just yet) , I have just recently became able to purchase a coin like this one and would like some feedback on what a fair price for it would be. Now I already know that something is only worth what someone is willing to pay, its that I just dont want to pay 600 bucks for a 400 dollar coin, I know that it list for like 650, but I havent been watching this coin for months and dont know if its sliding down or going up..

and info is greatly appreciated .