Corel Painter for $100? Painter costs about $400 actually.
Its a beautiful app, a scanner and some nice watercolour paper costs less.
Zbrush costs ~$600, includes paintstop (a corel painter like app).
So buy photoshop (a nice app for sketching too), or download gimp (its free). Buy a scanner too.
BTW digitizers are the greater partners of digital sculpture apps.