We rented a four-person surrey bike at Boardwalk, and it was $18 for 30 minutes. I'm not sure but I think it was $30-32 for an hour. I don't know why OKW would be different, but of course it could be.
They had some unanticipated restrictions concerning children below a certain age (?). They are not allowed to sit in the pedaling seats. Our 9yo had to sit between us in the front, and our 6yo had to sit in some sort of basket. But it was still fun and well worth the cost. We went around Crescent Lake three times!