Also (for Northy) got some Yoko prices as well.
195/50/15 AO48's semi-slick in either S/SS/Med compound are either £220 for 2 (delivered), or £410 for 4 (delivered).
A006T 190/580/15 (basically 195/50/15) which are FIA approved cut-slick tarmac rally tyres work out at £260 for 2 or £500 for 4.
And, if your going to use either slicks or semi-slicks you really will need some serious suspension work to get any gain from them.