The price for a gondola tour is fixed by law. This website gives you the current tarriff. HOWEVER, it is still important that you agree the price before you get in the boat. Gondoliers have been known to charge much more than this. And during off-peak times you may even be able to negotiate a slightly lower fee.
At the time of writing the price is 80 Euros for a 40 minute tour during the day. An extra 40 Euros for every additional 20 minutes.
An evening tour starts from 100 Euros for 40 minutes with an additional 50 Euros for an extra 20 minutes.