well, we were in Venice last October, there are many points along the grand canal to get a gondola ride, they all charge about the same, I think we paid about $110 Euro for 45 minutes, and that was during the afternoon, prices are higher at night, but it was worth it to my wife, was always a dream of hers, me, I could take it or leave it but it was nice, just pricey, more so now with the worse conversion rate.
Vaporettos probably go in canals the gondolas don't, they are pricey too, I guess everything in Venice is pretty expensive.
I'd rather die while I'm living than live while I'm dead
Just to clear up some terminology - Vaporettos are quite inexpensive as they are the public water transportation. They are just like a public bus - they have specific routes and stops. You can buy tickets for one ride, full day, or several days. Water taxis are private and are like land taxis - they only pick up you and go from point to point. Water taxis are expensive.
The Gondolas will sometimes go on the smaller canals in Venice where the vaporettos can't go.
This site has info and links for getting around Venice. getting around Venicel