The dealer will tell you that the Rays option is £1000 on top to the wheels that come with it, so for their true value you need to find the price of the standard wheels then add £1k. And obviously they do a discount for getting them as an option, they'd have no hope in hell of selling them as a £4k option!
Buying them singularly they sell them for far more as they are (supposidly) harder to come by and as a single item they will always charge you more. Just look at the deals Phil can get a full set of Nismos for, its far less. Second hand prices reflect that. Thats why I'm keeping my Rays as track wheels then have them refurbed when I come to sell