RS Turbo at £400 is crazy money, not a limited enough edition to command that price.
I sold one about 4 months ago for £160 and thought that was enough.
The only model I think ever worth big money £500+ is the Biante Dick Johnson RS500 with gold wheels as that was very very rare, although there are several newer versions by Apex models which are pretty good too.
As Jimmy said, a lot of newer models are more limited than they used to be, Otto only does limited runs and Sunstar too now, hence rise in prices