I've been PMed a couple of times by Americans asking whether they should exchange their US Dollars into another currency before travelling to Cuba so here's a little explanation...
The best way to convert US Dollars into Cuban Convertible Pesos (CUC) can appear confusing at first, but it's really just simple Grade 5 arithmetic. Here's the deal:
1.) Exchange Rates: All the internet currency exchange sites (like coinmill.com, xe.com, oanda.com, x-rates.com, etc.) are useless for real budgeting because they only give mid-market rates, ignoring the buy/sell costs that you'll be charged at the Bank or Cadeca in Cuba.
Here is the ONLY website that gives you the exact exchange rates that you will receive at the Bank in Cuba:http://www.bc.gob.cu/Espanol/tipo_cambio_METROPOLITANO.asp
2.) US Dollar value: This NEVER varies. $1 USD = 1 CUC minus the 10% surcharge and minus the buy/sell commission that is charged at any financial institution anywhere. In Cuba it's about 3%.
Bottom line: $100 USD = 87 CUC. Period. This does not vary.
3.) So, the ONLY calculation you have to do in order to decide whether it's worthwhile exchanging your US Dollars into a foreign currency like Canadian Dollars, Euros, etc. is to call up your bank or exchange house and ask them how many CAD, EUR, etc. you can buy with $100 USD, then using the link above see how many CUCs that will put in your pocket.
If you can get a great exchange rate on the foreign currency then perhaps it makes sense to face the hassle of the double conversion. If exchanging to the foreign currency only gives you back a slight advantage over the written-in-stone 87 CUC then obviously it's not worth the trouble.
It's that simple.
Have a gas in Cuba!