I was just walking around Walmart and I know it's not a good place to get bikes but they had a bargain bike section and they had a genesis hard tail 29er that a guy returned because the chain kept falling off because the tire wasn't set correctly for 120. There was also a genesis full suspension bike with 26's that was 95 because it had a busted shift cable.
I thought if the frames where ok maybe I could just run one of those and upgrade it as I go.
Over all, you will end up with some ok parts on a junk frame.... depending on how far into it you get. Your better off spending the money up front and just ride the bike. I got a higher end Kona seven years ago, I ride 2 to 5 days a week the only thing I have ever done to this bike is re-lace the rear wheel, replace the tires, and replace the disk brake pads. this bike has about 3000 miles on it, im guessing...I stopped counting at 1800....
Guess it depends on what you want to spend and how and where you are gonna ride it. Spend $400 to $600 at you local bike shop and be able to take it for service and buy parts. Spend $ 100 bucks at WalMart....service involves just swapping for another $100 dollar bike.
Hope this helps, i'm pretty passionate about mountain biking and it is easy to get discouraged when things don't work out on the trail. You also want to get a bike that fits you, too small or too big and you are gonna hate riding it. These are all things to keep in mind. I hope this helps...."thumbsup"