Dave's site has a good driving directions page here. This page has links with directions to the port from various locations. There's also a "maps" link at the top left. Follow this link and see the map of Cape Canaveral (top of the page), which shows where the Budget office is.
It's only about 3 miles or so. You basically drive south on 401 from the terminal. You then get on 528 east/ A1A south, and the Budget office is just ahead on A1A.
The maps on Dave's site will give you a better idea.
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