You can eat well and pay a lot of money, you can also eat bad and pay a lot of money.
There are few different options. Eating at your casa particular, state run restaurants, private restaurants (paladars), peso restaurants, and on the street. Understanding the differences is important.

Casa Particular
Most casa particulars will make, breakfast, lunch and dinner for a reasonable price. Breakfast, usually $2-3, consists of eggs, coffee, fruit, bread, juice, etc. Dinners can cost from $6 –12, and many will prepare any dinner you like as long as you order in advance so they can go shopping.

Restaurants and cafeterias

These can offer a wide variety of styles and prices.
In general, any restaurant that is not in a private home is state run. In recent years they have advanced the level of service and variety of styles.
At one time, all of the restaurants would have a large menu but never have any of the items. Many of them are still like that. You can tell the difference by the quality of the décor and waitstaff. There are many very inexpensive peso restaurants, but it can be difficult for a tourist to pay the same the price as Cubans. In other cities it is easier to walk in and eat, but in Havana they will tend to hustle you by either trying to charge you dollars instead of pesos or by handing you a menu designed to deceive tourists. You can tell the difference between prices in pesos by how many lines are in the dollar symbol. The peso only has one line through it and the dollar prices have the usual two lines. In general fried chicken or a pork dish with salad and rice will cost a Cuban between $10 and $16 pesos, but some restaurants will try to charge a tourist anywhere between $5 and $10 dollars for the same dish.

Paladars

Private run restaurants are one of the legal ways that Cubans can make money. They tend to be good but expensive. You can negotiate for a better price if you like. They usually have more items than are on the menu so you can ask if they have steak or lobster. Most of them are quaint with no more than 10 tables, and they tend to have good service. Many of the hustlers will try to get you into a paladar as many are hard to find. Always look at the menu and negotiate a price and trust your instinct. It will be more expensive if someone brings you since they get a commission. Also be careful of hustlers wanting to eat with you unless you want to pay for their dinner. Paladars close and reopen often, although fewer new places open than those that close for good.

Street food

The street food is cheap and generally safe but not great. You can get everything from ice cream,pizza, fried chicken, rice dishes, and more for anywhere from 10 cents to $2. It helps to get pesos first but these places can be good for changing money as well. Most of these places can be found on the street in doorways.