A good suggestion is to look at buying a second hand bike or one that is a few years (2-3) old from a shop. That way it should keep the prices down a lot so its not a massive cost at the start.
Any more common brands such as DK, GT, Redline (if you can find an older one) should be a good starter.
used is good. New wise... Can't beat a Redline Proline. Redline is on life support right now so you may be able to get one discounted well. think its the best bang for under 500.