Maybe, since the bike rentals are a private enterprise and not an attraction that would have a set fee. Bargaining in much of China is a way of life. As likely though, is people not remembering exactly what they paid - I know I can only say I enjoyed biking the wall very much, but couldn't tell you today exactly what it cost.
Also also the time of day for the rental and the weather likely affects the rental cost. If you're renting in the middle of a hot day (or maybe an especially cold day) when the rental place has a lot of bikes on-hand, they likely would be more agreeable to a reduction in price. I rented at a prime rental time, in the early evening just as the sun was going down, the temperature was very comfortable and the other bike rental place had just shut down, so the rental at the South Gate was the only one open into the evening, with bikes going out as fast as they were being returned. Not a situation where bargaining is likely to do any good.