I believe, the maximum brokerage commission is 1.8% for trades of a value below Kshs 100,000 and 1.5% for trades above Kshs 100,000. The additional levies are for NSE, CDSC, CMA etc.
The exception to this is on odd lots, which used to be a minimum of Kshs 100 regardless of the value of the trade. I am not sure whether this is still the case.
As for the minimum, brokers are liberty to charge whatever they deem to be OK with them. Theoretically, this can even be zero. Obviously you have to trade pretty high values/frequently to get lower than the standard commission rates.