For under 400$? Uh good luck finding a good bike for under 400$... Go to Walmart or Target and get a fuckin' cheap Mongoose(they have better bikes, but they mass produce cheap shit for Walmart) if your price range is that low...
http://www.walmart.com/catalog/product.do?product_id=8399258
Don't get me wrong, this bike is pretty bad, but it's a decent bang for the buck. You could even go cheaper and get a shittier bike. If you want a better investment... you're looking at spending maybe 600+. I'd go to a local bike shop and check out some other options. It all depends if you just plan on doing it casually or if you really want to get into it.