1999 $25 dollar Proof Gold coin.Is this coin a “very rare” coin? On Ebay it’s “BuyItNow” price is $2,355?

I was recently looking at currency items on Ebay, and I came across a 1999 $25 dollar Proof Gold coin that was for sale, and noticed it had a "Buy It Now" price of $2,355 dollars. I am fairly new to collecting coins, so I was hoping someone could please possibly help me out with a question I had about that coin. Is this coin a "very rare" coin? Is it because it's been graded by chance? Or is it a rare coin as far as the year? I have searched all over the web trying to find this out, and realized several sites that deal in gold coins order prescription drugs had several $25.00 gold coins for sell, but the 1999 coin wasn't available in their lists, they had surrounding years, but skipped 1999. Is this coin a "very rare" coin? I would greatly appreciate your help & response to this, It's driving me crazy trying to find out an honest response by searching the web, It interest me because i believe i may also have this coin in a collection i have in storage, but mine has not been graded. Does having the coin graded greatly increase the value as well? Again, I would really appreciate any help on this, and look forward to your response. I Would Appreciate An Urgent Honest Response! The Link To The Coin On Ebay is - http://cgi.ebay.com/ws/eBayISAPI.dll?Vie...