I searched extensively before my trip for a reasonably priced (<$100) place to stay in Nairobi for my last night in Kenya. I didn't want to spend much as I had to leave for the airport at 5am the following morning anyway. I found unlike Johannesburg, there is not much depth in accomodation in Nairobi and it is mostly expensive.
Whilst in Nairobi, however, a driver informed me of this hotel and it was the perfect place for a cheap one night's stay. It's well priced at ~26 USD for a single ensuite room per night. Nearby appears to be a karaoke bar as I could hear singing (quality of varying degrees!) for a good portion of the night but for the price of the room one can't complain. Across the road is a 24 hour shopping mall with a supermarket, ATM and eateries. As this hotel is right downtown, it is easy to walk around the city. I read Nairobi was very unsafe to walk around but I found it to be fine during the day, even by myself. Just be aware (if you need to change money/currency) that all banks and foreign exchange bureaus in Nairobi close from 1pm on Saturdays and do not open at all on Sundays.