I don't know the cost - in 1999 it was $10 round trip! - but I do know it's not automatic. You have to call Mears at least one day in advance, possibly 24 hours, to reserve. They'll tell you what time you're being picked up; they drop you off at the Hub at Universal; and they'll give you a card with a phone number to call two hours in advance of when you want to go back.
The trip TO Universal is often in vans, but on the way back they were using buses - so even though I was told two hours when I called, when I got over to the pickup location I was able to get right on a waiting bus.