There is nothing wrong with the $300 bikes. They'll do you well if you plan cruising around town, maybe hitting some easy trails. You don't need to spend 600 bucks to go biking. But think carefully about how you plan to use this bike because if you wanted to hit some good trails, maybe try some jumps, then these cheaper bikes will fall apart.
At about $5-600 you start getting into respectable bikes that can handle a trail. These are the real entry level mountain bikes. When you are at the bike shop take a close look at the bikes and compare 300 ones to the 600. Lift them up, check out all the components. If you want something respectable, keep saving.
Buy the bike at an independent bike shop - it's the only place to buy them. You'll get a good bike, good advice, and good support this way. Check out what's available in your town