On our first day after our 4 hour walking tour of Venice, we were completely drained and ate in the very first that we came to when we left our hotel after changing for dinner. The restaurant was the Bonvecchiati in the Hotel Bonevecchiati and it did the job very nicely.
On our second night we got a recommendation from our hotel and they made a reservation for us in the Ai Mercanti. After a little struggle to find the place (it was down a little hidden street) we had a an enjoyable meal with quite possibly the best plate of pasta that I have ever eaten. Pasta, oil, garlic, chilli & parmesan - so simple but so so good!
We had heard that it was expensive to eat out in Venice but did not find it to be so. There was lots of little sandwich bars where you could people watch and get an espresso for about a euro. There was also an option of ordering a pizza for less than 10 euro when eating out. I guess the best thing to do is check the cover charge before sitting down - one place down at the water was charging 6 euro cover and stay away from San Marco Square when eating!