You can get a visa to Armenia upon arrival in Yerevan, at the airport for 3000 dram, approximately 8 USD/6 EUR for 21 days and for 15000 dram, approximately 41 USD/30 EUR for 120 days. In general the procedure of obtaining visa is quite simple and it lasts about 20-25 minutes. In case your visa dates are expiring soon then it should be prolonged (500 dram approximately 1.6 USD/1 EUR per extra day).
Visa can also be obtained on the Internet at http://www.armeniaforeignministry.com/eVisa/. The combined application processing and e-Visa fee is $15 for 21 days and $60 for 120 days. Only credit card payments are accepted.