You can use the yellow cabs available round the clock for 250000 Rials (25 USD) to get to downtown destinations in Tehran. Since Tehran is a big city, you may be charged more if your destination is the northern parts of the city.
Ask the taxi station for the fare and use yellow cabs only. Do not use the cars around especially at night. For other airports which are located in the city, you can use other cars during day. Make sure you agree on the fare before getting onto taxis or cars.