There's not many " must see " roads in the southern half of Florida ( and not a whole hell of a lot of them in North Flotida either ). The above referenced route from Orlando to Key West is about as good as you'll do to miss the major slab. I don't know when you plan to go but wait as long as possible to avoid the heat issue in South Florida.
No such thing as a cheap place to stay in Key West...I just got back from there yesterday and spent $235.00 on a room. Stop by and see the Coral Castle in Homestead Florida when you pass through.