Sadly, Argentinian cab drivers are a very diverse bunch. Many many really good ones, but many many crooks too. Avoid the touts at AEP, and avoid the overpriced remises, too. Head for the marked taxi stand. A ride to Microcentro will cost roughly 25-35 (depending on traffic). If you're going to Palermo is much less. I've had touts quote me 60 pesos, then I proceed to laugh at them. Beware.