For bug-out, probably an external frame pack is best choice. If money is tight, check Salvation Army and Goodwill for Kelty or CampTrails. Have gotten excellent packs for under $10 at resales....

Not a whole lot of realism if you're planning to bug-out on foot; UNLESS you are going someplace in particular where there is gear, eqpt, and supplies to meet your needs. You could transport gear fairly well with a frame with a fold-down shelf.

Spend some time resaling and see what turns up. Good idea to have a couple of daypacks and maybe an internal frame pack as well. If you have a mountain bike, you could likely rig up some kind of frame to lash packs to the bike and that way push the bike with several packs mounted onboard.

If you intend to bug-out to some kind of wilderness area, maybe you should consider putting out caches and pre-positioning them, at least food & water etc to meet your needs on the trail.

A big sedan or truck, especially a full size Suburban makes a great Bug-Out vehicle since you can sleep in it. Take a set of tire chains and you can likely go anywhere.

Good luck!