I'll throw in my 2 cents on this one as I'm a wearer of both premium and cheaper denim, as well as a lady with, *ahem*, generous thighs
1. As Rae said, cheaper and premium denim are usually the same thickness. I find that all of my jeans (Joe's, CoH, Goldsign, JBrand, Guess, Gap, Zco) eventually wear out in the spot you're talking about.
2. Suz makes an excellent point. I've found that my jeans last a zillion (rough estimate) times longer when I rotate them. For you, this might mean buying mutiple pairs of the same style. I duplicated my favorite Joe's and marked the tag of each one with a different number, so I would know I wasn't wearing the same pair twice in a row. I did this because, like most people (I hope), I don't launder my jeans every time I wear them. Speaking of which...
3. Laundering makes a big difference in the life of your jeans. Are you washing after each wear? Unless you're getting dirty and sweaty, you probably don't need to. I find that every 3-4 wears is just fine.Also, I like to wash all of my jeans in cold water and hang them to dry. The less you stress the fabric, the less likely they are to shrink and wear away in the thigh area. I'm also a bit obsessive about my laundry habits, so I use a perfume and dye free powder, as it seems more gentle on fabrics in general.
4. Finally, the repair. I've found that if I check my jeans regularly, I can see when they start to get a little wear and tear in the thighs. I take preventative measures and patch them before all hell breaks loose. I buy the small iron on patches you find at fabric stores, which can also be found in most grocery and drug stores, and place them on the *inside* of the pant leg. A hot iron and a few minutes later, and my pants are ready to go
Of course, this is just my crazy denim preservation and repair regimen. Your mileage may vary!