I live just south of Sanford and I ALWAYS take 417 south vs. I-4 when heading anywhere towards Disney! My GPS will tell me its about 3-4 minutes longer but the GPS isn't looking a traffic on I-4 so in effect you'll make better time taking the 417 and difference in miles is negligible in my opinion.
The Expressway Authority has a "toll calculator" on it's website that will let you calculate the exact cost based on entry and exit points and # of axels.
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