You are going at a very popular time of year. It seems the rental companies know when they can have high prices and when they can't!!! Drives me nutty.
A car service to, and from, Sanford to WDW resorts, will run you around $200, plus tip. I usually can find a rental car at Sanford for less than that. But, you have to be diligent and keep trying. And you may find that the cost doesn't go down!!! If you're going to be stressed about it, you may be better off just booking a car service.
I have used Tiffany Towncar in the past with good success. Their rates are around $230 right now, but I think there are coupons out there that will take a little bit off the total.