Depends on how you want to live. You can use Western facilities, restaurants, food items, etc. and pay similar costs to London. But you can also live on a couple of dollars a day buying locally produced food at markets, sticking to non-imported products and things not designed for immigrants.

Accommodation inside Nairobi is fairly expensive, again similar to western prices, but you can get much cheaper accom outside the city - we just rented a big house with 1 acre of land for $500/m. And travel is very cheap, often < $1 for a 1 hour mutatu ride.

If you're primarily looking for a cool place to work from or a digital nomad kind of lifestyle, you'd very likely do better in one of those places. Only consider Nairobi or elsewhere in E. Africa if you have a specific reason or interest in the region.