Tampa is not that bad of a drive. You could rent a car for the day and drop it off when you get to WDW. Use National or
Alamo to drop off at the Dolphin or car care center. HOWEVER.....construction along I-4 is an absolute nightmare! It will take almost 2x the normal time to travel that stretch of highway. We do not fly into Tampa because of this. Our vacations are too short to waste that many hours in the car.
When you are going to WDW? Have you tried all of the different airline sites, the travel sites, expedia, orbitz, etc. I recently started using
www.kayak.com to see what is out there for my dates. Southwest (if they fly from your home airport) has some great rates through early September. How about the other discounted airlines, airtran, transmeridian, jetblue? Have you tried flying from a different airport near home. We always look at three different ones. We have a 45, 60, or 120 minute drive to the three different airports, but sometimes, the savings is worth it.