I have been to the Riverside museum, and I must say that I was really suprised from the start, parking was just one pound, which allowed a stay of three or four hours, entrance was free to the museum, the cafetaria was higher in prices to average, but offset by the free entrance, I don't really see why people complain about it.
Even parking that I'm used to here in The Netherlands would be 5 times as high depending on where you are going.