Definitely avoid charging for CM - that will put a total damper on community enthusiasm. The better approach would be to rationalize builds - we don't have to have nightly builds. IMHO even weekly builds would be plenty.
Weekly builds would be fine if the nightlies never broke. But nightlies often do break. So you end up using yesterdays or the day before. But going in back in time with weeklies pushes you back much quicker.