For machine costs true. The big savings is the labor cost for rebuilding PC's. Using ghost it takes close to an hour per PC, with VM snapshots it is only minutes. Multiply this by 10's or 100's times a day and the labor cost savings is huge.
A few years ago at the 'Mercury' Conference the guys from VMWare gave a presentation on a product they had which would do EXACTLY what you are looking for. It hooked in with QC and when a test was launched it would fire up VMWare and the Image you selected, then run the test, and remove the environment.
Not sure if it is still around... but worth asking VMWare or HP about it.