The pdf is free and you can get physical copies from Lulu for a $9.99. I am looking forward to the complete Mythras core rules being released in the next month or so.
Once again Loz and Pete have demonstrated unerring instinct and excellent judgment in their handling of
RuneQuest Mythras. A generic, lightweight d100 rules set
(based on the fantastic RQ 6 no less) is exactly the platform needed for developers to launch their own settings and to bring new players to Mythras.