I'll admit when I first heard of the concept of a BOPB I started from Doug Ritter's lists of necessary gear, which has almost nothing to do with brands. From those I was able to *assemble* ~75% kits from things I had on hand, and gradually supplement it with careful buys of things I knew I would need. Strangely enough I remember two of my first purchases were plastic and duct tape to shelter in place in our bathroom, cutting those to fit and be ready to hang. These days that's a remote possibility, but it was inexpensive and started me towards better preparedness.

But starting from Doug's lists you can have a checklist of things on hand and things you lack, and work from that. A monthly budget can help, and most places I've seen recommend focussing one month on food, the next on clothing, etc etc. Probably one of the best moves was getting my wife to think strategically about food, such that when she sees a staple on sale, she will buy more of it, and it will go into our pantry as emergency provisions. We eat what we prepare for, although there are a couple cases of MREs still just in case. Having a partner in crime has yielded a 1yr plus supply of toilet paper, soap and some other essentials - priorities...