It depends on what you actually want to do with it. If you want to ride single track, you need to consider the condition of the fork. Even lightly used, the forks from that many years ago won't be great. Then again, if you are actually looking to ride off road, $400 retail for a new bike won't get you much, either.
Edit. Just did a quick google search of the M50 from the mid to late 90s. Pretty entry level bikes back then. I wouldn't pay more than $50 or at most $75 for it.