Actually the taxis in Yerevan are plenty and they are relatively cheaper than those of in most of other countries. For the time being the price per km is 100 AMD, which roughly is about 0.18 Eurocents. The minimum fare is 500-600 AMD. Normally, within the limits of Yerevan you can reach any point for a maximum of 1500-1600 AMD or 3 EUROs.
There are things that a foreigner should take into account: although the local public is very welcoming, some taxi drivers may abuse the fact that you do not speak Armenian and do not know the city and try to overcharge you. If you are staying in a hotel, ask the staff to call a trusted taxi service they are cooperating with. Generally these taxi services would not abuse as the hotel has influence on them. If you are in the middle of nowhere and are looking for a taxi, be careful to stop a taxi that passes by. The taxi should have clear signs that it belongs to a real taxi service (taxi name on the door, a sign on the top of the car, and a green light visible from the windscreen). When you get in the car please look for the counter and check that the driver resets it before he takes the direction.
If you have a smartphone, you'd better install some checked local taxi apps like Yandex Taxi or GG Taxi. They have trusted services and you can easily track the route through your phone.
If you arrive in Yerevan Airport and there is nobody to pick you up, you'd have to take a taxi to Yerevan. Do not give in to persistent taxi drivers that are fishing naive visitors right in front of the Exit gates of the airport. Before you come out of the gates, in the left side of the gates you can see 2 stands: one for Airport taxies and the other C-Media taxi service. Please approach any of them and ask to arrange your transfer. It normally makes 2500 AMD or 5 EUROs to reach the center of Yerevan from the airport.