I agree with Katys Grandad.
According to their website, Alamo charges $70 a week (Okay... $69.95...). Plus tax (about 13% I think in Florida).
If you really can't rent one from this site, and feel that you absolutely need a satnav, I'd check out the prices and buy one: I don't know where you live, but here with Xmas coming you can get really good deals on those, just make sure you get the right maps [msnwink].

That said, driving in the US is very easy, you'll certainly be fine without a satnav.