If you have an old computer lying around, here's this for a suggestion:
http://www.maximumpc.com/article/features/cheap_and_nasty_how_build_open_source_server

Granted, it's for turning an old or cheap PC into a NAS box, it's really not that difficult to take those instructions and figure out how to make a Mac equivalent (assuming, of course, that you have no old PC's but an old Mac or two.) Just a money-saving thought.