R/T. There was one time a couple years ago when I saw a miraculous fare to Cairo for $600 including tax. Getting a ticket from Addis on Egypt Air or Ethiopian Air would still bring the grand total up to about $1000, though, so yes, you're dreaming...it can take 24 hours to travel there from NYC! Realistically, how cheap could it possibly get? When I originally looked into getting to Ethiopia that route (only if I could find another good rate to Cairo) it didn't work as well with my limited time off due to flight schedules...would have ended up with only 14 days in Ethiopia instead of 16, and those extra days are so precious. And doubly sad, I have never seen a ticket under $900 to Cairo since that crazy fare a few years ago (or before, really...that was a really unusual price).
As far as I know, the only inexpensive place to fly in Africa from NYC is Morocco (I saw $550 tickets just a few months ago), and you can't get to Ethiopia from there cheaply enough to justify the detour, unless you have loads of time for a different type of adventure.
