Venice can stretch a tight budget, and the best way to save money that I've found is to "picnic" for lunch and/or dinner. Picnicking is techinically not allowed in Venice, so keep a low profile (don't spread out your blanket in the middle of the square and chow down). We often found a bench or out of the way set of stairs to sit on.
Gastronomias, snack bars and alimentari shops are pretty easy to find in Venice, even along the Lista di Spagna. You can easily snag a huge calzone and a bottle of Coke for L6,000. Add to that a piece of fruit from the market or one of the stands along the Lista di Spagna, and you've got a cheap and filling meal. Pizza to go is also a good option - see if you can find a place that heats the pizza up on a sandwich grill rather than nuking it in the microwave (though such places are becoming harder and harder to find). Finish off your meal with a gelato or a pastry from the mouthwatering shops near Piazza San Marco.