It's about 600 for the retrofit including fitting the elements, replacing the climate control panel and coding. There's a vietnemese guy that does it - offers his services on ebay and does a very good job by all accounts ..
But this is obviously very spendy considering the initial costs - I'd recommend waiting for a car with the seats in the first place Rog!
Previously: 2003 Peugeot 206 1.6 8v | 2006 E90 320d M-Sport, 19" BBS CH, Full Ice-cold JL audio install, August 2010 Total BMW 6 page feature car. | 2003 Nissan 350Z GT Coupe 286BHP
Now:2010 E92 LCI 335d M-Sport