Some of it may also depend on what you want it for, your riding style, and where you're drawing the line between cheap, reasonable, and expensive. For myself, I'm on a Raleigh Mojave 4.0 that I got for about $400. It's basically the next up from the entry level Mojave. Overall, it's pretty good. Probably not the best, but I maintain it and keep things working well.
I have, however, concluded that I'm more into riding fire roads and other dirt roads than singletrack trails. Some of that may be because the geology where I live creates a lot of chert regolith that leads to trails consisting of fist-sized cobblestones and "baby heads." I loathe grinding over these long stretches of rubble. The well-worn sections over exposed bedrock or plain old dirt are a different story, though. Those are great.
Anyway, I had been looking at a cyclocross bike for the fire roads and such. One store was trying to sell me on a $1,100-$1,800 Specialized Tricross. For the money, it was a pretty good setup, and I could see you get what you pay for in a lot of the components. But another store recommended I just get some 700c rims and put them on my Mojave frame with some more cycloX-type tires. He was saying that the dirt roads around here are rough enough they'll destroy a cycloX in no time. I appreciated the advice and lack of a sales pitch like the other guy gave me. It was a plus that this was the LBS I bought my Mojave from.
I mostly ride roads during the week, and just today found a good round-trip route that takes me through some gravel/dirt road. I've concluded, however, that I think I'll stick with a mountain bike adapted for a road bias, keeping the 26" rims with slicks with just enough tooth for gravel and dirt. Some of the county roads are heavily travelled, and I like having the advantage of being able to get off onto the shoulder regardless of whether or not its paved. Many of the country folk here see a jersey and immediately paint a target on the person's back. It keeps me out of their way and earns a few brownie points being able to get off to the side... especially if an oncoming car would keep them from going around.
So I figure when the Mojave has seen better days, I'll stick with a mountain bike frame, but go with something with better components and a little lighter than the Mojave 4.0. It's aluminum, but still pretty heavy. I've hefted a few Cannondales at a local shop that are a lot lighter, but run around $600. For $1,000, it seems like overkill.
So, going back to the original point, some of it may be a balance between what's it worth to you for the price they're asking. For me, $400 was good for a solid entry-level one to get my feet wet and fitness built over a couple of years. When it comes time for a new one, $600 seems like a good baseline for build, better components, etc. Especially since I've sunk probably an extra $100 into the Mojave for a better seat, clipless pedals, better tires, etc. But around $1,000 would be my threshold for "more expensive than I need."