Like $80-90 for the two piece steel set (black powder coated) or $185 for the three piece aluminum set. Both include a smaller alt pulley to make up for the lost power. The three piece set includes a third pulley (same size as the stock one, but it is aluminum). It looks nice to have the billet, and it might weight a little less a free a fraction more hp.
I have the two piece set and love it. No alt problems at all.