I suppose you got off the airport bus and then went to Plaza Nueva.
It's only about 250 metres so as long as you haven't got loads of luggage it is better to walk.
You weren't ripped off because the minimum fare is 3.71€
Here are the normal prices. (correct in May 2011)
NORMAL TARIFF - TARIFA 1 - 07:00 - 22:00 (Sunday to Thursday) and 07:00 - 21:00 (Friday)
The initial price is 1.40€.
The price for each additional kilometre is 0.76€
The minimum price is 3.71€
Waiting time is 0.32€/minute
For a full price list click on this linkhttp://granadainfo.com/taxiinfo.htm