Is 800$ a month enough to live in Addis Ababa?

Dear,
It depends on what you are looking in life. Have you noticed how much a house rent in Addis Abeba? Ho and check in this website .
You also mentioned travelling through the land, do they offer you a car or thinking renting a car?
Please sit down and negotiate with your employer About the additional benefits you may get additional to thebasic salary you mentioned.
Of course you do not mention your qualification and the type of job.
In my opinion this salary you quoted, for a foreign national without any knowledge of Ethiopia is unacceptable salary given that things are getting expensive every day .

Try to get your boss up to 1,000 a month, then you will be good

Check out this site for cost of living  www.numbeo.com

Hello dear sir
the salary proposed is more than enough to live with in a budgeted standard but it is not enough to travel through out Ethiopia unless you save some.

Depends on how you live, USD 800 in black market will fetch you something like ETB 20,000.

Lets say you rent a villa house somewhere in Ayat region which is outskirts of town, rent will be ETB 10,000 (minimum) you left with ETB 10,000 , now if you are planning to travel across Ethiopia and again you won't be eating local food from day one (as it will take time for you to accustom to local flavor) I seriously don't think ETB 10,000 is enough.

Now if you choose to live in condominium (studio apartment)or service of some villa then your rent would be something like ETB 4-5 K, in that case I think for single person ETB 16-15 K is more then enough to live in Addis Ababa.

This all is valid if you are travelling alone, with family USD 800 is strict no.