A somewhat cheaper (but less capable) alternative is a toner and probe set used for telecommunications work. I use a Harris Pro2000:
http://www.sacramentoelectronics.com/harris/pro2000.html Kill the power, and clip the toner on one end of the run you want to trace. The probe can then follow the wire through standard walls, floors, and ceilings with little problem. Works best with NM and K+T, but with earphones or a quiet work area you can trace BX or MC runs. Metal conduit it can't do, unfortunately.
