There is the $100 walmart/acadamy bikes, made without regard for quality/weight/etc. And then there are the $1000+ bike store bikes.

That said, if you look at kids bikes at the local bike shop I think you will notice something. Those bikes tend to be just as good as the adult versions (good components, strong frames/etc) but they tend to cost under $250.

I think it says something.

I'm more of a commuter rider, so I don't know how bad the stresses of frequent hardcore mountain riding can be, but my current bike is a Trek hardtail mountain bike that I paid ~$520 for new at the bike shop, and it's served me well for years. Perhaps I should have said $500 instead of $400, but I'm pretty sure I've seen unsprung MTBs from good brands selling for $400 at my local.

Edit: I realize that a $400 unsprung MTB is not at all suited for a serious mountain rider. But I do think it would do for someone who's on the fence and doesn't want to spend thousands on a hobby they might not like. That's why I said "entry-level"--if you turn out to love mountain riding and wreck it in six weeks, then you know what to be saving for. If you find you don't like it, you haven't lost too much.

That's not quite accurate. Here's a random example:

http://www.rei.com/c/mountain-bikes

You should be able to find plenty of decent hardtail low-end mountain bikes in the $500 - $900 range, particularly if you wait until the off-season to do your shopping.

My personal advice would be to buy your way into a decent frame and upgrade components as they fail. If there's one thing I found from several years of avid mountain biking, it's that parts wear and break much faster than they do in road riding, giving plenty of upgrade opportunity. :-)