@kenmac, right on the money. If you set up Cell Broadcasting Services on your phone, you can view a log of all available rates over a day.
It's probable but virtually impossible to obtain a rate of 3/=, in Nairobi town centre during the day. And Safaricom uses the 'probability' to appear 'cheap' - face it!
This 'dynamic discounting' was first launched in Ghana by MTN/Ericcson and has won many awards. However, if providers were transparent (to their detriment), they'd actually publish the average rate on the service which for Safaricom I guesstimate at 6.**
Safaricom's strategy is not cost - that is YU's domain.