A hybrid has a more upright geometry, a little more heel clearance,possibly come with rack mounts, and less aggressive head tube angle [not as "twitchy"] than a mtn bike.Most mountain bikes are sized to accomodate a suspension fork of 80 to 150mm.The fork adds significantly to the cost of the overall bike and if the price is cheap so is the fork which doesn't matter much for paved riding. A lot of folks like riding mtn. bikes because of the geometry and more upright seating [over a road bike] but change to smooth tires in the 26" x 1.5" size for around $15 each.
The most critical issue is shifter function on a used bike after structual integrity of the frame,even if the shifters work fine plan on putting a new chain and cables in the near future.
Police auctions and thrift stores are another place bikes can be had pretty cheap, take a cycling buddy with you so you get an understanding of what is available.
Good luck and let us know what you get.