A jam group an hour from here does a workshop at the beginning of each monthly jam. PM me if you would like me to hook you up with their leader via e-mail. I believe regular jam members just take turns doing workshops.
At our newish monthly jam, we are paying a pair of workshop leaders to come to our jam. If it goes well, we will do this quarterly. We have to pay our leaders, so we are charging jammers a slight fee for that month's jam.