Havana is a surprisingly expensive city to stay in; if you stay in hotels and eat in restaurants it can work out to be nearly as expensive as other popular international destinations. The problem is that Cuba has a dual economy, if you could live on pesos it would be incredibly cheap. Sadly, as a tourist this is virtually impossible. Most peso hotels won't take foreigners or, if they do you have to pay in CUC. If you are on any kind of a budget it is advised to stay in casas particulares; it is much cheaper, often more comfortable and the food a recurring theme in Cuba is almost invariably better.
ATMs are not too hard to find in downtown Havana, but bear in mind that American credit- and debit-cards can not be used in Havana. Note that even credit cards issued in countries other than the USA may be issued by a bank whose parent company is a U.S. corporation. In this case, the card will not work as the parent company is bound by U.S. law. Even banks wholly owned by non-American companies may have a policy on blocking Cuban transactions in order not to compromise their US business. Always check with your bank or credit card company before leaving home to see if your card will work in Havana. Also, the ATMs do not accept MasterCard/Maestro but are marked to accept Visa.
You can withdraw money from your MasterCard in a couple of exchange offices. There is one in the basement of the Hotel Nacional, but expect quite steep service fees.
Exchanging US dollars in a CADECA Casa de cambio will incur a 10% penality. Sterling, Euros and Canadian dollars can easily be exchanged at Cadecas and do not incur the same fee.
