Taxis in BA are not cheap. They start at 9.50 pesos (almost $2 USD) and then go up. From Palermo Hollywood to San Telmo it cost about 75 or 80 pesos ($16 USD) - and it takes a long time. Instead learn the subway system and go anywhere for a flat 2.50 pesos ($0.50 USD) - now that's what I'm talking about. It took less time for me to get from Palermo to San Telmo (Plaza de Mayo).
But during rush hour, the trains get VERY crowded. So avoid the rush hour times (and besides, you're on vacation, you shouldn't be up that early!).