As a first time backpacker I got quite a bit of sticker shock for appropriate backpacks. I never knew they would be so expensive, so when my cousin introduced me to this great discount gear website I was incredible relieved.
Usually when it comes to discount sellers, you will find cheaper brands or lower quality products, but I was pleasantly surprised to find some of the top quality brands I was originally looking at. I've come across Osprey and Deuter packs ranging from the smaller 20-something liter packs to full 75 liter packs, and a good variety in between. When I was shopping around at REI these were selling in the high $180's and over $200. The exact packs were being sold through this site for $80 to $160!
SteepandCheap.com is the site that my cousin suggested and to really take advantage of their savings you need to understand how the site works. They offer products for limited amounts of time in limited quantities. So they may be selling the backpack that you want for 2 days and only have 50 in stock. The key to this is to regularly check the listings and act fast when your items are for sale.
I highly recommend going to whatever outfitter is closest to you (for me its REI) and trying on all the backpacks and getting them fitted, then keep an eye out for the ones you like on the site.