I am an avid collect/restorer/rider of vintage road bicycles, many of which are featured on my website - MY "TEN SPEEDS". That said, I have recently become interested in vintage mountain bicycles.
Since my interest in relatively new, I do not know all that much about the vintage mountain bicycle. My guess is the one presented is of little value in the vintage world. Though the Mongoose is a nice brand, the bicycle is, as someone else pointed out, tainted with department store taint. To that add that the one you are interested in is really pretty darn new and not quite vintage in my opinion.
Were I buying the bicycle, I would pay up to $40.00 for it, and not a penny more. Were I selling the bicycle, I would expect to get between $100 and $150, and that is assuming it is all cleaned up and working properly.
Please keep in mind that any price anyone suggests is little more than a guess about value. There are a host of things that impact
Vintage Bicycle Value, many of which have little or nothing to do with the bicycle itself.
Anyway, good luck with the bicycle and enjoy the ride. Though not a highly collectable piece, the bike will, none the less, be fun to ride and increase in value when it does become a vintage machine (about 12 years from now, best guess).