Ohhh ditch the fivers! The real deal is in the poor side of the tourism (as a way of speaking of staying in hostels hehe)

Central America is believed to be cheap, Ive only done belize and Guatemala but so far they are as expensive as México (for me i have my country to compare but if you are in the states or Europe Im sure you will find Central America cheap as hell).
Party Island in Belize is San Pedro! Also you can do San Blas in Panama! Im sure there are some in Costa Rica, I guess I will finf out when I get there!