Little difference, it's the internals and the mapping that cost the money AFAIK so you're looking at the difference in kit costs. Which in the scheme of £15k isn't going to make a huge amount of difference.
The car is a £30k+ car and you're going to be adding more than 50% of it's new power. So it's not as bad as it sounds. It's also not been done over here as much as it has in the States so it costs more to do.
Don't get lightweight ones then, get coloured.
Personally, I don't want people looking at the bolts, I want the wheels to be the feature. So I'd keep the nuts plain and clean and go for the best wheels.