No reason for a Wally World or low end frame for your build. Craigslist is loaded with very nice chromoly framed rigid mountain bikes with decent components for $75-$125. Just do a little research so you know how to identify the better models of the rigid mtbs. Mine was $125 and has a nice ishiwata double butted frame, chromoly fork and decent components and juts needed some cleaning and lube.

Good luck on the hunt.