Taxi drivers in Prague are notorious for trying it on with anyone. To see how bad it is, Prague's mayor recently dressed up as an Italian tourist and hailed a taxi for a short ride, and was promptly overcharged five times the official rate.
The starting rate should be about 25Kč, and 20Kč per kilometre after that (34Kč/25Kč respectively if you hail an honest one on the street). From central Prague, a ride to the airport has risen to about 600Kč; a short ride in the city centre should be about 130Kč, a ride to one of the surrounding districts up to 200Kč.
There's really no need to agree beforehand on a price (a good way to get ripped off) if you simply phone one of the reliable taxi companies listed below who will usually have a car ready under ten minutes. Else, hail only their cars if you must.