Hi,
Try to check out Nigist Tower. I used to stay there for two months. Their website is: nigisttowers.com/?page_id=3. If I am not mistaken, single room rate would be something like US$ 60.
As for the costs of living in Addis for a single person, I think it would depend on which part of town do you want to live, as the biggest chunk of your expenses will go to paying rent. For example, in some areas like around UNECA or Bole, expect that you would spend around US$ 1,500 per month to rent a house (if you stay at a guest house, you might pay something like US$ 1,800). But, if you stay a bit far, you could keep the costs down to US$ 1,000 a month. If you could share an apartment, it would be even better. Then you add costs of meal, which will be probably not too much, like US$ 90 per month. Add your transportation cost, utility bills, etc, like US$ 200 - US$ 300. If you total the figures up, you might end up with something between US$ 1,200 to US$ 2,000 per month (depending on your type and location of your accommodation). I hope this information helps.