Mike, s2 seats vary in price but a good set in leather will set you back between £500 and £700, I got a brand new set for £500 but had to drive to blackpool at a moments notice.
I think the big advantage is in what John has said they are made for an almost identical car and there weight is only about 5.5kg per seat compared to about 30-35kg for a normal sports seat
http://cgi.ebay.co.uk/Lotus-Elise-S2...QQcmdZViewItem
This is like mine