A few years ago I did a Grand gathering, and we needed the van-taxis. We never paid more for them. I have also been a small party in a van-taxi (a sedan would have been fine), and never charged more.
Nicole
I got the earlier joke. Some years ago, maybe 2005? We took a van cab from AK to POR. The fare before tip was over $35. So to me, that seems higher than what other guests are reporting. In that case, it was worth every penny over taking the bus to DTD (the only choice we were given at that hour.) Perhaps quoting Robo was not the best choice to pair with my response.
As for the other comments on the value of taking a taxi, it really depends what your priorities are and what your point A and point B are. If taking a taxi is going to spare me half an hour or more, then yeah, it's easily worth $20. But lots of times we've gone to take the bus/boat and found a bus/boat waiting to depart, in which case a taxi would not have saved us really any time. Other tiems we've waited 30min for the bus to arrive - in which case a taxi woudl have been much faster and less nervewracking.
Frequently I've posted that time is money on the DIS - usually in the reverse sense. Many 'money saving' tips posted here don't really save money, IMO, because they take up too much time, or require a very bold move. ('free' lemonade anyone?

) When I've figured the cost-per-waking hour for us to be at WDW, it works out to be roughly $20-25 per hour. So if it doesn't save me over $25, then it isn't worth an hour of my time. Here, we are talking about shelling out $20+ to save less than an hour - often only a few minutes, if anything. (Uncertain because of the complexities involved) (having a taxi waiting for you, walking to the taxi stand, where the taxi is able to pick you up and drop you off, how long untilt he bus arrives, number of stops, etc.) If you are late for an ADR/meeting someone, your priorities may be different.
And a few places are not really accessable via taxi - the IG gateway and the entrance to MK. So in some cases, taking a taxi will cost money and more time. Taxi's will only pick up/drop off at the resorts in front of the lobby. In many cases that's near the main/only bust stop. In other cases, like CSR, the main lobby may be quite a bit further form your room than the nearest bust stop - or even the main bus stop. (which is usually close to the resort's dining). Mobility devices are another huge aspect to transportaion.
But then....there's also the emotional aspect. Some folks strongly dislike buses, or get anxious over waiting. Some folks feel the monorail or boat are an essential part of a trip to MK. There's no way to price those feelings.