The current rules and prices are clearly stated here
From the website of the Ethiopian Embassy in London.
Click on Consular Services, then VISA and it is all there.
BUT US citizens should note they have different rules and prices....
This is what it says on WHO is eligible for a visa on arrival.
Tourist visas can only be issued on arrival to:
A....Foreign nationals who are coming from countries where there is no Ethiopian Mission
(NOTE THAT THIS more generous and helpful statement is fairly new ..September 2017)
B.....Nationals from Argentina, Australia, Austria, Belgium, Brazil, Canada, China, Denmark, Finland, France, Germany, Greece, Ireland, Israel, Italy, Japan, Korea Republic, Kuwait, Luxembourg, Mexico, Netherlands, New Zealand, Norway, Poland, Portugal, Russian Federation, South Africa, Spain, Sweden, Switzerland, United Kingdom, United States.
Please note that visas on arrival are only applicable to tourist visas. Business visas cannot be obtained on arrival. You are required to secure a business visa in advance at your nearest Embassy or Consulate.