>>2447061Depends what you want. A rigid steel mountain bike from the 80s or 90s will cost around 100-300 dollars on craigslist. It will likely need new tires, tubes, brake pads, and chain. "Rigid" means no suspension. Old bikes with suspension are not going to have good suspension, but the rigid ones are great bikes. These early mountain bikes are a lot of fun to fuck around on and can handle mellow to intermediate singletrack just fine.
That said, if you want to do crazy stuff like ride fast over rock gardens, jump off drops, that kind of stuff, then you'll want a more modern bike with suspension. You'll be looking at 1,000+ dollars even if buying used. If going with a hardtail (no rear suspension, just front suspension) you may be able to get by with a little cheaper, but it's definitely going to be a lot more expensive than an old 90s bike.
When you say 'cool tricks', depends what you mean. Typically, people don't really do tricks on mountain bikes, other than the really crazy elite-tier riders who do flips. For the most part, the people doing tricks on bikes are riding either BMX bikes (at a skatepark or a BMX track) or dirt jump bikes (at a dirt jump track). If tricks are your thing, I would recommend looking into dirt jump bikes.
If you want to buy a used 80s or 90s mountain bike, check craigslist. If you want a more modern mountain bike with suspension, check
pinkbike.com/buysellThere is also some more info on the different types of mountain bikes at
topnigger.ueuo.com 
