lorna hit the nail on the head. The cost of the jeans is paying for the brand and the styling (cut, fit, wash, etc.) and that's about it in most cases.
If you think about it, there is no way for a pair of jeans to be of a level of "quality" purely in terms of durability and workmanship that makes them worth $300. If when you say quality you are referring to the fit and cut and wash/whiskering/etc. as well as the craftsmanship that goes into them and their ability to withstand wear and tear, then it's really a mixed bag.
Buying R&R, 7FAM, Diesel, TR (and all the rest) is paying for a brand name. Of course, with all the money they make selling those brand names, these companies are able to invest a lot more money into the cuts and washes they produce than a company like American Eagle or Bluenotes. Furthermore, 7FAM and R&R do have very well-documented durability issues. But I would argue that TR and Diesel are not of spectacular quality, they just shine in comparison to the easy ripping of R&R and 7FAM. I also like their cuts and styles a lot less, TR in particular -- the only people I know who buy and like TR are people I speak to online, none of my friends in "real life" like the brand at all. (Boring washes, ugly pockets, weird baggy fit...)
If you want to talk about actual quality of denim and workmanship, I think you have to look at PRPS and similar brands, who take the time to craft jeans out of selvage denim on Japanese looms out of African-grown denim.
Nudies are quite hardy, too, so they are worth the money in terms of longevity; but they are pretty trendy and unimpressive in the style department.