Yes it is good. There are many other doing that business, including one guy supported by GoK through the Kenya National Council on Science and Technology. So don't go into it thinking that you are the first.
Many matatus already have the gadgets.
Going prices are between 15k and 35k. You will have a hard time convincing people to pay monthly fees....Many players charge a one off installation fee and that's it - it includes the tracking s/w
Nothing is real unless it can be named; nothing has value unless it can be sold; money is worthless unless you spend it.