Good news if you're planning ahead, the visa fee that was cut by 50% in April 2009 is set to stay at its reduced level indefinitely. If you're getting a visa in advance, it will cost $25 plus any passport mail/handling charges (or £20 or €20 if you're getting it in the UK or in the Euro zone). If you wait to get your visa on arrival, the price is $25, usually in US dollars cash only. Under 16s are exempt. There's no real disadvantage to waiting to get your visa until arrival, especially if you download your form from an embassy or high commission website in advance – for example in London – and fill it in in before you join the queue at Nairobi or Mombasa. Visas on arrival don't require photos, either.
Kenya High Commission, London
Kenyan Embassy, Washington DC