Maximum Rates are set by the city of San Diego, but Taxi companies can choose to offer cheaper rates in order to attract customers. The maximum you'll pay in San Diego is $2.90 for stepping foot in the cab, $3.10 per mile and $24.00 per hour. Fares to San Diego International Airport (SAN) are using a slightly different logic (all companies use the same rates), and a slightly different scale ($2.50 + $2.70/Mi.+ $22.00/Hr).
Trying to shop for the company with the best possible price is a losers game as companies change their price scales on a regular basis, and the best price may depend on your fare: companies with a low flag drop rate will often have a higher mileage rate, and the other way around. Either way, price differences between companies are small, and probably not worth the time you would spend shopping around.
A typical fare within city limits will set you back $10-15 on average.
Many airport trips will cost you less than $25 because most popular locations are found within a few miles of the Airport: $13 for the Convention center, $20 for the University of San Diego, $25 for SeaWorld - $46 for UCSD though. If you're headed for a Mexican escapade, you can take a cab to the border for about $70.
San Diego does not have many taxicabs cruising up & down the streets, hailing cab might leave you waiting for a long time. A good strategy is to locate a nearby major hotel or attraction as they will often have a Taxi stand, or get Taxi Mojo on your iPhone or Android to order a cab when you are on the go.