Spec-Ops THE pack can be found reasonably right now. Their desert camo version (with extra pouches) is $90, and the foliage green version $100. While they may be a little bit bigger than you are looking for, I thought that I'd throw them out there.
http://www.specopsbrand.com
http://www.greyghostgear.com/ also has several Tactical Tailor packs on their website for under $90. That would be another option..
If you look (on that famous auction site) for the phrase "Tactical Assault Gear back pack", you can find TAG's excellent 3-Day Assault Pack for under $100 right now. They are listed as being coyote brown in color, but I think they are actually khaki. Here is video on TAG's Chaos Pack.
http://www.youtube.com/watch?v=ZrO6XeNRQpc The two packs are identical in every way, except that Chaos Pack doesn't have PALS webbing on the outside.
The Camelbak Urban Assault and Urban Transport are two more options. Both can be found online for $20-30 more than your $100 budget.
I'm sure there are other packs out there, but those are the only ones that I can think of at the moment that fall into your price range.
Finally, check the EE here on the forum. You never know what you might find on there. Heck, you might stumble upon a lightly used Blue Force Gear Vulcan or Overlord pack, RAID pack, or an ATS Tactical Cobra pack for sale..