It makes me laugh when I read other reviews shouting out things like "rip off" and "tourist trap". Nobody is making you eat and drink there. If you don't like the prices then move along.
We had had a long day walking around Venice and went to St. marks Square at about 7pm. We had eaten late so didn't want dinner yet. We knew that the prices would be high because of the location, we were told straight away about the music charge and we looked at the wine list and saw the various prices. We ordered a bottle of red and enjoyed a leisurely hour people watching and listening to the music. The waiters were attentive and very polite and it was nice to just chill out. And then we had a second bottle and spent another hour here. We were brought crisps and snacks throughout and the bill including music charge came to €90. Yes it's more expensive than some restaurants located down an alley way but you are paying for the ambience, the views and yes the music. We were happy to do that and we had a great evening.