Hi Ska - it is actually minimum salary, not cap. players may demand as high a salary as they like (though maybe no manager will sign them). BUT players may never have a salary below the minimum for their category - you cannot hire Heyheuhei for $1 per game.
Have another look at the wiki page (still subject to changes) for more information
http://wiki.warlight.net/index.php/Manager_League_Season_Beta