So just to clarify, in SD taxis have a price fluctuation deeping on where you are going within the city?
Do they not have meters inside?
They charge by the zone. No meters. I have not used cabs in SD for quite some time. It used to be RD$120 minimum charge, across town (e.g. Gazcue to Herrera about 160) now with gas/LPG prices up I guess what MAR paid RD$140 is the minimum now... across town about 200 pesos.
Call Apollo Taxi, Aerotaxi. Rueda taxi is a lil'bit cheaper on longer trips.
Never use a hotel taxi as they will charge you double to tripple the rate. If you flag down taxi that belongs to a company (e.g. Apollo) they charge you the same rate as if you call in to ask for a cab. If you flag down a private cab not affiliated to a cab company expect to pay 20-50% more.
If the quoted rate does seem too high to you, tell them to check the rate (verifique la tarifa por favor) and listen closely what the dispatch says.
If you do a multiple stop trip they should give you a discount and always ask for them to call the base to check the rate. The rate usually includes a short wait time (e.g. 5 minutes) longer wait time is usually paid at a rate of 30 mins = 150 pesos. If you do longer distance trips or multiple stops they usually charge less for waiting (if not excessive).