I used the Mears cab fare estimator for Epcot to a few locations close to DTD (it's not a choice in their calculator) and get around $11-12, without any tip of course. I cannot speak to the accuracy of their numbers or if it's entirely up to date.
It cost us about $18 with tip to go from MK to Beach Club in 2008. I would budget $20 per cab ride and that would probably be good, with possibly some budget left over. Disney is very large, so it's going to be a range of fares - most likely from $12 - $25.