An Extremely Short Guide!
Radiant Silvergun was an extremely popular game in its day. This game is not "RARE" as stated in every single auction you'll see. The reason for its inflated price is purely because so many people want it, and users from asia see this as an opportunity to make some serious money.
You are much better off looking at japanese websites for a 2nd hand copy, which can be less than £20, and paying for shipping at another £15 or so, than buying from an eBay seller for over £100.