Tim, I’m not sure what your definition of “dirt cheap” is but you can look on Amazon or eBay and get some packs that would be okay for around 30.00 or 40.00 bucks depending on the size you need or you might try Goodwill or other resale/donation/pawnshop stores. You can also try Facebook and a new place called Letgo. I would also check with the Boy Scouts and to see if they have some good used backpacks that they would be willing to let them go for cheap or maybe a local National Guard post might know of some used gear. If you’re a vet, I bet you could get hold of one of the vet organizations and they could hook you up.
I hope this helps in some way, good luck in your search.
U.S. Navy – Semper Fortis

When it comes down to it there is little to gain by signalling your capability to others.

We’re talking group dynamics here. People are going to be targeting others by using strength in numbers. Anything short of a sizable group of prepared people obviously capable of immediate and coherent retaliation will not be much of a deterrent.

Should you miraculously not find yourself in such company in a survival situation, you as the capable looking guy will be targeted first, not only for equipment but to strike at the head and scatter the sheep.