Should be cheaper for a steam gauge C172 or not too much more for multi or complex. That should suffice to get someone some cheaper hrs up through Commercial Pilot, Instrument - Airplane, Multi.
There is very little grey area. Currently, in US:
There is an FAA approved version of X-Plane for commercial use. This is often sold with the hardware and cockpit setup. Usually includes an instructor station for system integrity check and monitoring. This setup gets an FAA certification/designation as a Basic Aviation Training Device (BATD) or Advanced Aviation Training Device (AATD).
Logs on the sim must be maintained and a CFI or CFII present to sign the logs of the student pilot.
The logs should able to be cross-referenced, if necessary. This is needed in case someone is suspected of adding extra ink to his or her personal logbook or if someone claims sim credit as part of the application for an airman certificate or rating.