Went on this early August.
Cost was GBP55/equiv. of EUR86. for 2 adults and 2 kids.
I think you could get a combined bus tour/river cruise for a lot less, so I would suggest you shop around, as this experience did not command such a high price.
Also the tour was due to last 75 mins, but I reckon it was nearer 60 mins in on peak traffic.
If you are determined to go, I would suggest you book in advance at peak times, as we only got on because we had booked in advance.
Guide was not great, not funny, and generally you felt he was not making to effort. Would not recommend.
A similar DUKW tour in Dublin lasts longer, and costs EUR60 for 2 adults and 3 kids.