One alternative to an expensive and complicated taxi meter and price regulation is the "zone card" currently used (illegally) by gypsy cabs and (legally) in Washington DC. Draw a map of the city, divide it into regional neighborhood zones, and charge based on how many zones you have to pass through to get from point A to point B. If it's LEGAL to provide service on that basis, you can print the zone map on the outside of the cab (as well as printing it on the back of your business card or whatever) so people can compare fares before they get in.

Zone map for Washington, DC.