From what I saw, accomodation is pretty expensive in Kigali...
I stayed at the One love hotel. I don't exactly remember the price, but it ranged around 25-30 USD for a room with a bathroom inside and a small terrace.
The manager, Daniel, is very friendly and helpful. There is also a small bar/restaurant on the premises that serves the biggest lump of ugali I have ever seen!
It's a bit far away from the center (half and hour if you want to walk and it's good exercise because you just keep on climbing!), but you can get around easily in moto taxis and Kigali is very safe, so there should not be any problem getting there and away at any time.