Despite its reputation, Venice does not have to be an expensive place in which to enjoy eating and drinking. It can be but doesn't have to be. There are plenty of top end restaurants where you can spend a fortune, and if you insist on taking a coffee in St Mark's Square then be prepared to pay €10 -15 for the privilege. Equally, with a little planning, good eating in nice surroundings at a reasonable price is perfectly possible. 
First, some general points and then I'll offer some suggestions. Firstly, Venice is not the culinary capital of Italy - even the locals admit to that. 
Like all cities, Venice offers a range of eating from the very expensive to the very reasonable. As a general rule, the area between San Marco and the Rialto is a tourist trap and the prices reflect this. That said, in the streets between the two there are a large number of open fronted cafes serving slices of pizza for a few euros as well as those establishments where you can buy a panini or baguette for the same price. Not a bad option for lunchtime eating. 
This area also has a number of restaurants that offer a tourist menu. I have eaten in them and they are not bad, but expect only fairly basic stuff like bolognese and lasagne. You should also be sure (and make it clear) that you want to order from the tourist menu and check that that's what you've been given. With a bit of diligence you should get a reasonable lunch for €10-15 a head. Wine, of course, will push the price up. 
Away from the bustle of St Mark's and the Rialto you can discover little cafes and bars, particulalrly in Cannareggio and Castello. My recommendation is to look for those serving cicchetti (a kind of Italian tapas pronounced chi-KET-tee). The deal here is to order at the counter selecting individual items such as ground spiced mince balls or tuna in breadcrumbs that have been deep fried, bruschetta, chicken wings or perhaps hard boiled eggs topped with anchovies. Each piece costs around €2-€3 but you can have as much or as little as you like. Again, great for lunch accompanied by a small glass of wine or two. There is some further information about some where to enjoy good cicchetti in the section about bacari (bars) in the drinking section. 
If you can tear yourself away from the very expensive establishments in St Mark's Square, taking a coffee in one of the small cafes is a good option. As with most continental eateries, the price is cheaper if you take it standing at the bar rather than sitting down. Also, if eating, food served at a table outside can be more expensive than a table inside. 
And, of course, like every city in the world a Big Mac or a Whopper are there to be found along with the omnipresent Hard Rock Cafe. 
Mid-range restaurants are to be found all over and there is no greater pleasure than eating al fresco in Venice after a morning's exploration. Whichever sestiere you find yourself in, there will be a restaurant to enjoy but I would always check the menu. I'd emphasise that point: check and order from the menu. Some friends of mine made the mistake of sitting down at a little restaurant on one of the islands where the owner enquired, "Do you like fish? You do? Leave this to me". Eight courses later and they were looking at a bill that made the eyes water. 
At the top end, there are some absolutely superb places for eating at but the rule will have to be: if you need to know the price, you probably shouldn't be there. 
Venetian specialities to try include: 
Spaghetti Vongole (pasta with clams) 
Stuffed, deep fried courgette flowers 
Saor - a local sweet/sour sauce. Many dishes come with it including fish, particularly sardines 
Baccala - salted cod served as a paste or mashed 
Polenta - a maize based alternative to pasta 
Fegato - Calf's liver with onions 
Gelati! - Fabulous Italian ice cream 
and, of course cicchett (see above)
OK. On to the specifics, here are a few places I enjoy at the value end of the market:

As I said at the beginning of this section, you don't have to spend a fortune in Venice to eat perfectly well. Fast food joints aside (yes, they're to be found), there are some great little establishments offering both restaurant and snack food at good quality and very reasonable prices. We are operating in the €3-40 price bracket. 
I would recommend marking these on your map and planning any walks to take you close by. 
Arte della Pizza (Calle dell'Aseo, 1896 Cannaregio) 

Why am I giving away all of my secrets? Here you'll find the best value pizza in town served whole (huge) or by the slice. Always busy and offering fantastic value. Last time I was here I had a whole pizza which would have served 2 or possibly three and it cost me....€7. Noisy, cramped, not many seats but, come on, improvise. Essentially, this is a street front eatery bar with a couple of tables thrown in. 
Basically, pizzas come in two sizes: regular and huge but, for me, the best option is to buy a slice from a large, varied daily selection in the display for a few euros. When you compare these pizzas to the usual over the counter offering....well, there is no comparison. These are fresh, homemade and loaded with ingredients. 

Ristorante Al Vagon (Canareggio 5596A)

I've enjoyed some well cooked food here and been served by some fun and efficient waiting staff.

Hotel Malibran (Canareggio 5864) 
The food here follows a "touristy" formula but I like it because the staff are such a friendly bunch. It is fantastic value for money and only two minutes walk from the Rialto Bridge. Prices have not increased since lockdown and so remain reasonable
To find it: Come across the Rialto Bridge to the San Marco side where you will find the statue of Goldoni in Campo San Bortolomio. Walk past the statue with Goldoni facing you. There are two exits from the square - take the left one. Now follow your nose. You go up some steps to cross a small canal and keep going forward until you come to the church which is painted an orange/pink colour. Look to your right where the hotel is rather brightly lit in the evening. 
Al Muro (Rialto Market, Saturdays) 
Ok. I confess that I've never been around to sample the apparent delights of this open air food emporium. It appears at the Rialto Market on a Saturday morning and I'm told serves a selection of really good food that makes it a feature of the Venetian weekend trip to buy fish, fruit and veg. Enjoy the market and then tuck in to some filling and tasty food prepared al fresco. Allow around €8 a plate with wine available by glass or bottle. 
Cantina do Mori 
This is a great little bar close to the Rialto Market. Pop in (as the market traders do) for a quick glass of wine from its extensive range. It also features fantastic cicchetti to go with your drink. Work on €3-4 for the wine and €3 for a piece of cicchetti. 
Pots and pans hang from the ceiling, service is "brisk" rather than friendly although asking for a recommendation tends to break the ice. There are (at the last time of visiting) only four chairs (an increase of one since lockdown!) in the main bar and a small shelf on which to rest your lunch. There are sometimes tables to be had outside. Not to be missed. 
Un Mondo di Vino (Salizada San Canciano, Cannaregio 5984)

A true locals bar included here because of its amazing selection of cicchetti. Highly recommended.

As always, I cannot be held responsible if your tastes don't match mine and standards may change.