Hard to imagine finding a decent mountain bike, new, for $200.
But if you're going to be sticking mainly to paved paths with little trail riding an entry level bike may be adequate.
I would never recommend buying a bike form a discount store. I always suggest checking with your local bike shops. The cost difference for a truly equivalent product vs. a discount store is usually not much and the service will be infinitely superior.
If you do end up buying a bike from a discount store, you'll be riding a machine that was built in the factory that was savagely beaten to the lowest price possible and then assembled by Wallyworld shelf-stocking monkey. You'll at least want to strip it down and reassemble it yourself before using it.