You'd need to define "average cache" I think your results might be region specific.
I'm about 5 times more likely to find a trackable than a pathtag in a cache around here. And there are only 3 other cachers who leave pathtags, so the picking is pretty slim. Chances of finding a trackable in a cache large enough for trackables is actually pretty decent - 25% or better, I'd say.
Thanks to you both. Someone could do the study regionalised, I guess... Does anyone know of someone who has actually done a study like this?