We just came back home from our 10 day tour of Venice/Florence/Rome the last week of September. Wow... it was great. As for the gondola rides the cost....depends. Basic cost at the time we were there was 80 euros, and went up as the day turned to darkness.
Our whole group (28 persons + Graeme our guide + accordion player and vocalist) went in five gondolas. The whole thing was just fantastic and final cost for each was 20 euros each.
if a number in the group do not wish to go, our RS guide said he would negotiate the best price he could - but everyone ended up going. The music and vocalist were really cool going down the narrow canals with the three to four story buildings compacting the music and voice - it was enchanting. Then we got out onto the Grand Canal and people on the canal banks were applauding the vocalist and it was cool. Well worth the cost.
If you want to go for a late night ride, do as Rick advises in his Venice book - negotiate a price first, then go for it. A great experience.