The cost of a single entry tourist visa is now £15 or $25
You can either get your visa beforehand at the Kenyan High Commission in London, or you can rock up in Nairobi and do it there. I got mine at the airport last time and it was perfectly easy (although renewing it three months later was slightly more problematic) but both options are fine. If you get the visa in Nairobi, I am pretty sure you have to pay with US dollars.
http://www.kenyahighcommission.net/visas.html
Visa Requirements
-A valid passport with at least one unused page for endorsement. It should be valid for at least three months at the date of entry to Kenya.
-A completed Visa application form (V1).
-One recent passport size photo.
-Travel itinerary or business letter.
-A stamped self addressed envelope preferably special delivery (for postal applications).
-Indicate your daytime telephone number.
Grandma - this is the one time you'll be the only one who doesn't need a visa!!