So I am a beginner trapper, but am always interested in data, this was something I have always wondered about with hunting, fishing and now trapping. I think the very first factor is not what bait or lure you use, but understanding your area and knowing where the animals are and what part of their movement they are in. For example this previous hunting season, I killed 2 deer, but I could have killed probably 30, and thats all with maybe 35-40 sits total. There was only a few times that I didn't see deer, I almost always saw at least one. Now I understood where the deer groups were, and kept track of their general movement based on what I saw.
Now on to trapping, I have looked far and wide and I am going to say that if you tallied up the total overall % for almost everybody it's somewhere between 0%-25%. However that curve isn't linear in my opinion, because I think there is a trade off and that is mainly how much time you spend on each set. Here is the big assume though, if an animal is going to walk by your trap, how much time you spent putting it in, will generally dictate your catch rate percentage assuming you understand what your doing. The hard part is getting the animal to actually walk or move by your set, everything works in cycles and so if you wanted to net the highest percent, you would stay in tune with the feeding cycles of the animals. Getting off on a tangent, but mine is somewhere around 10%-15% in creeks though I am sitting at a lower percentage. Not sure if that helps or makes it worse!