>>1391533MTB is an abusive activity to bikes, so there are some trade-offs here you should be aware of:
Bike snob route: Some bike shops will offer yearly maintenance packages at a discount for paying a painful upfront price for a good bike. If you just want to get out there and ride and not have to worry about fixing up your bike, definitely go this route. Quality entry-level hard tails are in the in $650 range, full suspension are usually just north of $1000, maybe $850 if you can find a good sale.
Thrift Route: You will need to become familiar with bike maintenance and repair, either buying your own tools or joining a community co-op like
>>1392210 suggested. Your bike quality will be poorer and parts will break and go out of alignment more often.
A perfect example of this: I bought a used 26" hardtail for $350 off Craigslist. It has a 3-speed front crankset, which is kind of bulky. Wound up getting bent on a log. I had to drop another $40 to replace it because the shifting was messed up -- and that was the cost repairing it myself, after buying a replacement and a crank puller (wound up using some EMT as a cheater bar, or else that would have been added cost too). On newer bikes, the gearing has changed so higher end bikes will have a 1-speed front crankset, meaning higher ground clearance, and less required maintenance.
I also think the type of terrain you plan on riding or that you have ambitions to ride in the near future should factor into your decision. If the trails you like to ride on are forestry roads (4x4) or gravel, go the thrift route. You can get by with knowing less about repair/maintenance, the bike won't take as much abuse, replacement parts will be cheaper. If you want to crush berms and not spend your weekends bleeding brake lines, be a freaking snob and have fun. And if you don't know what kind of trails you like to ride, then get off the internet, rent a bike, and ride.
