You can get comparative taxi, Uber, and Lyft fare estimates from RideGuru.com and I have found them to be pretty accurate. With either Uber or Lyft, you will get an actual fare quote prior to placing your ride request.
I'm not sure how Lyft operates in the Orlando market, but with Uber the price you are quoted will be the price you pay regardless of traffic, detours, or any other factors.
If you have more than 4 people, with no luggage, you will need Uber XL, Lyft Plus, or MinnieVan. MinnieVans do not go offsite, except to MCO for $150. Regular MinnieVan fares are $25 regardless of distance.
With the saturation of drivers in the WDW area, you should be boarding an Uber or Lyft within minutes of placing your ride request. At many venues, drivers wait in a staging lot and an electronic queue is used to relieve congestion at the actual pickup points. Many Disney venues have designated pickup areas as Joel referenced above, and drivers are not allowed to pick up in other spots.