Hi Paul,
Mental health is treated just like any physical condition when it comes to the medicals and they'll be looking at potential cost. As stormy said, it's worth speaking to a good migration agent and one who may specialise in medical conditions. Information that is useful is - how well controlled the condition is, hospitalisations, functionality e.g. working/studying (showing that the impact of the illness is small). I've known people get visa's who've been in hospital in the past under the act but their symptoms well contolled and working etc.,
Good luck