From the ads currently on http://www.gayromeo.com, the going rate is $50 per hour or $100 overnight!
Dollars are converted into CUCs with a 15% fee. You can't use dollars in Cuba or ATM on cards issued in the US. Money can be converted at the airport or at money Xchanges; hotel conversion is the pits based on what I've read recently!
I might buy Euros and convert; that way there's no fee charged although the Euros "kick the butt" of our dollars. Might get Canadian instead and do the same, but my best bet will be from Euros to CUCs.