Good afternoon GTIAIN,
I am fairly sure that the rear lights on the GTI are the same as any of the "Standard" MK5 Golfs, the rear lights only change for the Edition 30 and R32 the in terms that they are a tinted darker version.
As for prices I have recently called my local VW dealer to ask the price of the the D/S rear light cluster for the ED 30 and was quoted £62 INC VAT, as for the price of the whole set I dont know.
Thanks
Richard