The Blackmarket in Cuba is a very good way to get what you need and get it cheap, from Booze to Fuel, to Food, to pretty much anything else you might need.
Because of the communist system, they Cuban are on a ration system, so in order to get things they need, barter is very common, whuch also ties into the blackmarket. If you go, bringing things that you can sell is a good idea. Levis Jeans, Cargo pants, Electronics ( VCR, DVD, Game Consoles...) You will have to amke a contact you can trust, which I did not find too hard and that person will be able to help you sell your items on the blackmarket for American dollars.
I was at the end of one of my trips and had run out of money, i mean I had NO money...I ended up selling some of my clothes, and made some good coin, i had enough to cover a night in a private room, my airport tax leaving, and a meal the next day. I wanted to treat some friends to a nice meal one night, so I bought 20 lobster tails for 12$ American, cooked and spiced and ready to go. Fuel is about 1$ American for tourists, but a Cuban can get it much cheaper and you will pay about .50$ to buy from them.
The one thing that I would caution about is Cigars, ovioulsy one one the main exports of Cuba and a very nice treat to a tracv'la sitting in a cafe...The odds of you buying counterfit cigars is great, there are alot of them, and the look like the real thing...an amature cigar smoker would not be able to tell the difference, but a aficionado would. So buy your cigars in a shop, everything else try and get on the black market.
It might seem a little shaddy, but this is a way of life down there, the Cuban people can not just walk into any store and by something they need, most of the "luxuries" come from the black market. This is the same way of obtaiing items that is used in China, and the former USSR, and Germany before the fall of the wall.
Depending on the size of the town or city yuour in will depend on what you will be able to get amnd the cost of things will vary. Hangling skills will come in handy for sure! And be aware that this is illegal, so don't get caught!!!