Taxi rates are something like $3.75 for the first mile and $2.50 for each additional mile. It's at least 15 miles from the airport to WDW, or more, depending on which resort you're going to. That comes to roughly $41 one-way if you're lucky, more if the mileage is greater.
Tiffany is $80 round trip (list price). You can get a $5 coupon at their Web site, and if you have PassPorter Walt Disney World 2000 you'll find an $8 coupon in the back of the book.
When you add the extra comfort, the help with your baggage at the airport, and the grocery stop (be sure to request the grocery shop prior to your arrival), how can you compare?
If there are just the two of you, you can save money using the Mears shuttle. With the $3-off coupon that comes with your Disney resort confirmation it's $24 per person round trip. It's a bus ride, you don't get help with your baggage, and you should allow at an additional 45 minutes travel time in each direction.
Co-Author, PassPorter Walt Disney World 2000 http://www.passporter.com/wdw/