Yeah, if you have a rating 12 focus probably the best thing to do is just drop it. Even trying to sell it is going to result in someone trying to kill you for it. You don't want that kind of trouble in your life.
Now, as for a benchmark chart... yeah you're not really going to find anything I think. Not only is your question inherently affected by the "eye of the beholder" factor, rating isn't really the relevant factor here in gauging rarity/level of difficulty to obtain. If that is what you are interested in, then Availability is what's important. Foci Availability codes are derived from Focus Force, although different kinds of Foci have different formulas to compute Availability. Availability ratings, you CAN fairly objectively gauge, but I won't go into that here other than to say once you're beyond 20 or so, yeah that's generally territory where it's easier to kill someone to get rather than find someone willing to sell it.
Now, with regards to:
I was thinking about outfitting some NPCs with some cool enchanted gear
Remember that only magically capable NPCs can make use of foci. A street sam can use a katana weapon focus, but only as a katana. Its magical benefit doesn't work in his hands. If you want to give NPCs "magic gear", odds are what you want to do is use Alchemical Preparations: these are items anyone can use that have a one-shot spell in them. Rings that turn you invisible when you put them on, potions that heal you when you drink them, etc.
Now if you are intending on using lots of mages as NPCs, then sure give them foci. Just remember, Shadowrun ain't D&D (I mention this since you called yourself a DM). While yes, you're absolutely right to think ahead about potential problems with giving NPCs easily looted gear, Shadowrunners generally aren't supposed to loot the dead. If you take a Foci from a dead goon, the corp/mob/yaks/whoever won't come after you because of the value of the focus. They'll come after you for the principle of killing their goon. One of the LAST things you want to be doing is linking yourself to your body trail by hawking stuff you stole from the dead.