I couldn't find much about data plan pricing in Kenya, but if I'm reading this[1] page right, there are unlimited plans for the equivalent of about USD $10 per month. Really not that bad.
What kind of average family is buying an $80 cellphone? The majority are still going to use cheap used phones shipped in from the EU, or discarded by the richer members of their society.
The majority are using features phones(from edge-bluetooth-o.x-MP-camera to basic call-&text) manufactured by Nokia, samsung, lg,SE and Shenzhen. People rarely acquire phones discarded from the richer.
[1] http://kalahari.xtgem.com/internet_tarrifs_in_kenya