I have posted about this before, but a reminder that there are no longer any special discounts for domestic Ethiopian Airlines fights for those who bought their international ticket with the same airline.
However, as of mid 2011, domestic flight tickets bought in Addis Ababa (and presumably elsewhere in Ethiopia) cost around half to one-third of the price of the same ticket bought outside the country or online though the Ethiopian Airlines website.
In other words, assuming that you are willing to risk buying domestic tickets after you land in Ethiopia, it will work out a lot cheaper than buying them in advance, irrespective of which airline you used to fly to Ethiopia. The risk you take, however, is that the flights you want are fully booked, especially in the high season.
Bradt Ethiopia Travel News 
Great info, thank you! One question though.. Is it possible to pay in cash or only credit card?
I don’y know for sure, but Ethiopia is such a cash-based country, I really can’t imagine there would be any problem at all with paying cash
OK that’s a good thing, thank you once more! I’ll probably arrive in Addis on July 18th and, if possible, I would love to fly out to other cities too.