Quick answer: Venice runs about EUR 155 per day mid-range - noticeably less if you eat local and walk. Cheapest window: April-June and September-October - mild and clear; November-December risks acqua alta high water, and Carnival and August bring peak crowds.
What things actually cost
espresso standing at a bar
1.30 EUR
spritz in Cannaregio
4 EUR
cicchetto
2 EUR
vaporetto single
9.50 EUR
vaporetto day pass
25 EUR
gondola ride (official daytime rate, 30 min)
90 EUR
Eat well for little
Cicchetti at All'Arco or Cantina Do Mori near the Rialto - 1.50-2.50 EUR each, eaten standing with a glass of wine
Sarde in saor (sweet-sour sardines) in a bacaro - about 10 EUR
Spaghetti al nero di seppia (squid ink) at a mid-range osteria - about 17 EUR
Spritz with Select or Aperol standing at a Cannaregio bar - about 4 EUR (triple that on Piazza San Marco)
Tramezzini sandwiches from a bar counter - 2-3 EUR, the cheap lunch locals actually eat
Money mistakes to avoid
Day-tripping in for four hours - you will mostly see queues; stay overnight for the quiet early mornings and evenings
Eating at places with photo menus near St Mark's - walk 10 minutes into Cannaregio or Castello and pay half for better food
Not validating your vaporetto ticket at the platform reader before boarding - inspectors fine on the spot
Forgetting the day-visitor access fee charged on peak dates (about 5-10 EUR) - register online in advance if you are not staying overnight
Transport without the tourist tax
From Marco Polo airport take the ATVO or ACTV bus to Piazzale Roma (about 10 EUR, 25 min) or the Alilaguna boat into the city (about 15 EUR, 60-75 min). Inside Venice you walk everywhere; vaporetto singles cost 9.50 EUR, so a 25 EUR day pass pays off fast on island days.