@Yuoaman phew, it was my first time answering on a forum and I have to say I was a bit anxious over it. I'm happy you find my thoughts interesting.
I myself am a big fan of physiological costs - the bloodier, messier and thougher, the better. At the same time, though, a not physical thing can be equally interesting, if well managed by the writer.
So, I was thinking, what if using up one's will would lead to apathy and depression? It can get really nasty, with older Dynatheurgists becoming unable to get out of bed, or to take care of themselves properly, and maybe even suicidal.
Of course it's a difficult road to take: easy to fall into a lot of telling without showing, easy to fall into the bad teenage drama sort of thing - as a reader, though, I would be intrigued! It also gives you rather a lot of freedom with deciding how intense the effects are.