For a mountain bike I don't recommend a real expensive helmet. The reason is you are typically paying for it to be lighter. A little heavier helmet will protect your head just as well and save you a good chunk of change. There is a good chance that you will break your helmet, even if you don't it is a good idea to replace it every two years.

Here are my favorite mountain bike helmets.
Giro Havoc - $49.98.
Giro E2 - $89.98.

I typically buy last years models, much cheaper and pretty much the same helmet. For the last couple of years I have been using the E2. I'm very happy with it. Now if you want to spend some extra money check out the Atmos. That is what I use on my roadie. The extra ventilation is great and it weighs next to nothing. However, it does cost around $180.