I guess this is your lucky day as i'm just coming back from the Horn Of Africa ( Djibouti,Ethiopia,Somalia and Erithrea ).
Security in Addis Ababa Bole Airport is very strict but not the point where you can not take a toothpaste and some hand cream which is what i had in my handbag...(Removal of jackets,shoes,jewellery,belts...you know your normal stuff these days).
Ok,i may have slightly exagerrated with the jewellery but really my bracelets beeped and i had to remove them so the second time around i removed them and i didn't beep.And i guess it depends of the security machines because the first time you set foot in the airport you have to come throught security then the second time is when you are going inside the departure lounge to wait for your flight.
I took my visa on entry to Addis Ababa Bole Airport as when i was going to Ethiopia more than a month ago it was during the festivities of the New Millenium and all the Embassies were closed for holiday so i did some research to see if i could collect a visa upon arrival and to my fabulous surprise i did.it cost me $20 only.How cool is that.
And for my return to Addis Ababa from Djibouti,i just went to the Ethiopian Embassy in Djibouti and paid for my visa.
I didn't quite understand your second paragraph,do you need to have on hand $50 when you arrive at the airport???I don't think so and it is not my first time to Ethiopia as i go there regurlaly to see my family.
I hope that was helpful...Let me know if you need any more info.
From Djiboutici x