From everyone I've spoken with, they seem most sought after on older DMD games because the keyfob is sometimes quite rare. I think GnR is one of the hard to find keyfobs.
Stern keyfobs are very common so they aren't worth much, however, there are people who collect them. Stern sent me a large bag of keyfobs for pin a go go last year and when I announced it while promoting the tournament a few people emailed me privately asking if I would give or sell them a particular one.
I won a small bag of williams fobs at a tournament last year. If you need a particular one and I have it, I will "sell" you one in exchange for a $5 donation to Pacific Pinball Museum, Playland not at the Beach or Pinside (your choice). US Postage paid, or $6 donation for international.
Offhand I have fun house and Cirqus Voltaire. Would have to look at them to see what else I got.
Email me at jonnynacho AT gmail. Next three days are crazy so I may not return emails until Sunday.