Hi Vance-- I am not a professional collector but have some experience with this but can give some insite in to your questions:
How much do the permits cost and what do they cover- Every state is different but for instance, S Dakota USDA National Forest office offer special forest products permits regulate tree transplant collecting have size, type, amount and locational (state forest, BLM land etc.) restrictions often it is minimal--like 5 bucks per tree--each one has a tag. There are special set of rules for"commercial"collectors.
(
http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsm9_012514.pdf).
Have you ever had the Forest Service check your permits and harvest? Yes, and have heard of harrowing stories from others of being accosted by unrelated (ignorant and suspicious) law enforcement. In my case I had 6 Minnesota DNR officers detain me and my truck load of trees when I parked near a State park and was hiking--the trees were collected elsewhere on private property. I talked my way out of trouble. Also in Dakota the office checked permits and tree sizes with no problem. One frind in Dakota was detained by a sheriff for hours after a tourist called him in--he was fully legal but it watsed a lot of time. I have heard of one guy that once got a 5000 dollar fine out east. I believe most state and federal land using people are very suspicious if they see you digging trees up on public land and assume you are doing something gravely nefarious--I personally try to be sly or at least prudent.