how much it costs to extend an M visa and whether or not you can extend it for 3 months.

Ach, the extension fee is around CNY 160. The duration of stay you acquire won't be longer than the one you got previously. Whether you can change M to X depends on local entry and exit management bureau. However, according to what I read recently, it's almost impossible to do that. You have to get back to your country to reapply. Anyway, you can visit local bureau to confirm this.