I'm back.... At least for this conversation. I'm an SP and MUTA in the real world. I currently work in eastern Tennessee and previously worked in NY and NJ.
For an SP, the training varies as do the assignments. Most training is on the job and compensated. I'm know there are private companies that supply services to med schools. I'm sure the greedy ones would charge for training.
I teach and test physical exam for various medical and allied health professionals. For example, they'll get a lecture and cadaver lab time for a particular set of body systems then clinical time with the SPs to practice under our guidance. Later, they'll be tested on those exam skills and graded by the SPs backed up by video.
Other times, we'll play a character with a particular set of symptoms and clues. That character may be getting a history and physical exam or just an interview.
Sometimes the SPs are simply bodies while the professors demonstrate techniques. Sometimes we're tasked with operating the video recorders and acting as timer for testing sessions.
The MUTAs and GTAs where I've worked teach the appropriate exam using our bodies for practice. Generally, the students don't get tested, just guided to do things correctly.
Compensation ranges from in the teens per hour for the simple stuff up to 50+ per hour for MUTA/GTA work. Compensation will vary depending on location. Big cities pay more than out in the country. No, there are no volunteers.