A decent one can make bank. Neighbor had one, said he made $200+k on it a year. It was a big one, and the only one in the area. It was full service and staffed. I drove by it almost every day, and it was always packed, so I don't doubt his claim.
Judging by the condition of most of them, I'd guess the income isn't all that great. There were 6+ near where I used to live, and something on them was always broken, and often the entire place was out of order. I drove to all of them once, and went home with a dirty car.
People are sold on the idea that they are great passive income, but find they aren't that passive, and maintenance and lot costs are huge.