I agree with 45-70. Buy something nice, it will last longer enough to matter. I went cheap with my first couple of bags in college, but then I bought an Uphill/Down with a zippered computer compartment. That was over ten years ago, and I used it for everything while I was in school -- shooting, hiking, school, overnight trips, etc. I still have it, and until a couple of months ago, I was using it as the bag I took to work. The seam on the bottom of the pens/pencils/other small items compartment started to separate. I think I've gotten my moneys' worth out of it, but I'm going to repair the seam and pack it up for one of my kids' 72 hour kit.
I started using a (much) cheaper bag I had lying around after that seam split. The shoulder straps started to separate after only a couple of months. I got a new SwissGear bag at Staples, and I don't expect to be replacing it anytime soon. Yeah, it was a little bit of money, but that doesn't really cost more in the long run if you take care of your gear. Quite the contrary! If you buy quality, it lasts so much longer than cheap, the cost comparison is ludicrous. I paid 60 dollars for my new bag, and the same for my old one. The cheap one cost less than 20. If I was replacing a cheap back for 15 or 20 dollars every six months to a year, then over the past 12 years, if it was every year, I would have spent as much as 240 dollars, or three times as much as the first bag, if it was 20 dollars and once a year. If it was more often...
The point is, with any kind of gear, buy the good to excellent quality gear, even if it means spending a little more than you wanted to. You won't be sorry. "The bitterness of low quality remains long after the sweetness of low price is forgotten."