From what part of Florida? If you're going from Miami to Key West it will be about $230 round trip each. Have a look at http://www.cheaptickets.com - use MIA and EYW airport codes- or compare American and Delta airlines, they run the most frequent flights.
By road- If you are going from Miami you can check KeysShuttle.com this is a shared van ride that picks you up from the airport and delivers you to your door for around $75 each- 5hr trip. Also TransFloridian Shuttle does the same thing.
You can rent a car of course, but if you only want to drive down and then not use it the one way costs are high. It's about a 4+ hr drive from Miami- VERY busy traffic around Miami then down to Key West it's easy driving.
Key West is a small town, very easy to drive around, no major traffic. The downtown area is very compact.

Take a look at the electric carts for rental on the streets of Key West, a fun way to get around. Also scooters are popular.It is HOT there!