I don't think the answer is as simple as any of the replies I have seen. If you start with a cheap tubing set of straight gauge tubing, there is a lot less labor in building the frame. When you get into thin wall, multi-butted, heat treated tubing, you don't hire just anyone to weld (or in some cases fillet braze) your framesets. I believe there's also a heating process after the frame is welded to restore certain properties inherent in the tubeset. This would not be necessary with cheaper tubing. Obviously, you would expect better performance from a highly manipulated tubeset that was optimized for the specified application.

What I'm getting at is that the answer is two-fold. I'm also not aware of costly bikes made form cheap tubing. Do you have an example? I'll further point out that only one frame material will last virtually forever, and that's titanium. Contrary to what people might think, it's a relatively inexpensive material. Processing it is what drives up the cost. Machining requires very expensive tooling, and welding requires that the joint be absolutely contamination free, or the weld will quickly fail. If you were having a custom frame made to your specifications, and wanted it to last, you cannot find a better material than titanium. Argue that if you wish, but we weren't going to put up with the uncomfortable stiffness of aluminum, or the extra weight of steel for our tandem. This is a frame we will own for many years, and expect to rack up lots of saddle time. I've gotten totally off topic, but I'll sum it up by saying that I've seen people having heaps of fun out in the bush on rigid bikes that cost AU$300. It's not the tubing, or the bike, it's about how much fun you have. If you're my age, you probably had a Mongoose made from 4130, with 44x16 gears, and rode that all day everywhere. We're spoiled now.