I highly recommend the North Face Recon backpack. It fits your specs and is very well designed and built. I have had mine about 5 years now, and I still love it.
http://www.thenorthface.com/webapp/wcs/s...0GOLD%20RIPSTOPIt lists for $99, but you can get it on Amazon for $59.40. Ok, it is $10 more than your requirement, but I think it is well worth it (and I paid $89 for mine). I have had a number of packs over the years, and nothing was even close to this.
The Recon is an awesome pack.