Going on a tour by gondola through the canals of Venice is not cheap but it is surely very evocative.
We are going to list some general prices for you:

Daytime hire (of course with the gondolier)
50 minutes, six persons at the most per gondola EURO 73,00
For every 25 minutes exceeding the first 50 EURO 37,00
From h.8,00 p.m. to 8,00 a.m.

Hire service and tour by night
50 minutes, six persons at the most per gondola EURO 91,00
For every 25 minutes exceeding the first 50 EURO 47,00

If necessary you can get on a gondola also by using it as a ferry-boat (as lots of Venetians do) in order to reach the opposite side of the Canal Grande. The price is of about 1€. The most famous ferry-crossings are the ones between the Frari and St. Samuele and between St. Fosca and Rialto Market.