I guess the first question is how long you intend your BOB to enable you to bug out. I would guess a 3-day pack would suffice for most BOB's. The EAGLEŽ A-III ASSAULT 3-DAY BACKPACK looks promising, although for a bit more than you want to spend. Maybe the medium ALICE pack, used with or without the frame, for about $55 w/o frame.
I am actually looking into 72-hour BOB options too, although since I already have a large ALICE pack, I may just use that, with extra room for region specifics (cold weather gear; extra water) and/or extended bug-out times.
It's important to explore whether or not what you already have is viable for the intended purpose.

My question is: How does a person carry LBE gear and a ruck at the same time? I read somewhere that it is supposed to be that way, but I can't imagine how a heavy backpack (large ALICE) can be worn over a load-bearing belt and harness. Don't they both need to occupy the same hip space?