I'm not totally positive what the average salary is as we have several hundred employees with very different jobs. However, the entry level position for those aspiring to become guide dog instructors is probably about $30,000 at this point. The entry position has you working in the kennels and also with the vet staff and some basic work with the training department. The salaries go up quite a bit as you are promoted, but it is a nonprofit, so we aren't in the job for the money!
The high majority of any large service dog school IS going to be the volunteers, as all are non profits. While we have 250 or so employees, we have over 1200 volunteers.
Our dogs are completely free of charge and most large schools are the same (or they might have a very small fee less than $1000). This includes no charge to our clients for spending about a month being trained at our residential facility.
Our credentials are individualized to our school,but the International Guide Dog Federation has to approve of our process. In the state of California, where the largest guide dog school in North America is located, the state has requirements and there are colleges that offer training for this type of work.