First off, welcome pmccorm2005 to the ChiefDelphi forum! While we're not really into pitching machines, we do know a lot of things about a lot of things.
You see, we're a bunch of geeks (and proud of it!) that work with an organization called FIRST Robotics (
www.usfirst.org), dedicated to inspiring kids to better understand match, science, and technology. We build robots to help accomplish that goal. Kind of like wiffleball for the mind.
Anyway, to your question: If this is in fact an AC (alternating current, like what comes out of the wall) motor, the only real way to vary its speed is to vary the frequency of the AC going to it (and, as already noted, these start at about $100). BUT since it is already variable in speed, it is far more likely that while the unit runs on AC, the motor control is something else and not a variable frequency drive system. Meaning an external controller is more likely to just get the pitching machine angry and let out the magic smoke.
Before you start getting out screwdrivers and soldering irons, the first step should be to contact the folks who make this unit. They may have some simple suggestions for slowing the pitches down even further than what you have now. Ask for the engineering manager, or the technical support team, explain what you have and what you want to do, likely they'll be able to help. If you don't understand what they tell you, write it down, tell us, and we can help there.
Good luck!