Mears also has an estimator. It'll give you a ball park figure of the cost. If you are getting the a taxi in WDW, then only Mears and their subsidiaries are allowed to pick up passengers from within WDW.
Mears estimator can sometimes be way off. I think they use straight-line distance rather than road distance, which can be highly misleading in WDW because of the complex road network. TaxiFareFinder.com uses an actual calculated driving route and is much more accurate.