Mora's already been covered, its THE PAW knife to have in the $15 range.
For bags under $50 ...
1) Medium alice, used $12, with frame, $20, large with frame, $35. I'd recommend the medium with frame, I think its a good balance of size and extra room on frame to strap stuff below the pack. I tend to stuff too much into the large, medium keeps me honest.
2) For more $$, you can often find ILBE used on ebay for $125, or sometimes even $100, IMO, those a GREAT packs, awesome frame / load bearing, very durable, slightly heavy compared to the civy version because of the heavier duty material.
3) Looking to round out your cheap BOB kit? Hit up the dollar tree or dollar general, get a $2 metal dish to cook / boil water / eat in, $1 for a couple forks and spoons, $10 blue tarp (make a shelter), find some rope, $6 to $15 dollars for a used USGI foam sleep pad or blue walmart pad. Free = 1 liter empty bottles of gatorade, they fit great in the alice sized side pockets
. (note, most water pure tabs are designed to purify 1 liter of water
) $30 = USGI wool blanket, firesteel for $4, lighter, 75 cents. $15 for the mora .... DUCT TAPE ...
You CAN have a regular camping set up, pack, shelter and all, that will work for years, for under $100.
Now go camping and practice your bob skills.