There are more than 600 entries in the Devon Cave Registry but I havn't had time to do a more detailed analysis. I would think these are mostly caves but most of these caves will be grotty little holes of only a few metres. (This figure covers some sea caves as well)
Almost all of Devons larger caves have restricted or controlled access. There are only about 10 of these. The only one I can think of that is open is Prid so your 90% figure seems OK.
Most of the small grotty holes are open.
I don't know if this is any help
If I find the time I will do a more detailed analysis but don't hold your breath