Would this be of any use? http://www.nextgenhardsurface.com/showthread.php?358-Modeling-Cutting-Holes-Into-Flat-Geometry-Images Its for cutting holes but can be reversed
On the Maxon site the ZBrush subscription seems to be $359 annually. They also sent out an e-mail offering 50% off for perpetual license holders, valid until August 5th.