I just wanted to post my thanks to 'wahooflyer' for this informative thread. I am 21 and will be down in Florida for 10 days in March and was looking at rental prices of $350 - 500 dollars! After it was all said and done -> National ended up being the cheapest, for an intermediate sized car, at $190.25 all in, without any underage daily charges! I was also able to utilize corporate discount codes found from this thread:
Discount Contract ID's