I am a Canadian woman married to a Kenyan Man. I dont know how many cows your bride will be worth but I will check with my husband and see what he says. I imagine that because she is being married by a Western man, she will be traditionally deemed to be worth more cows then a woman married to another African. Also, gifts - and ;lots of them - for the family will be expected. these gifts dont need to be expensive or extravagant - the idea is that they come from overseas... you will be viewed as wealthy whether you are or arn't. When you see the poverty that many of your inlaws will be subject to you will understand why they think this way.
I would suggest cheap t-shirts - lots of them. Remember, she likely has 5 or 6 brothers and sisters and aunties and uncles with cousions everywhere. 30-40 gifts are probably a safe bet. Also, it sounds cheap but the dollar store with local paraphanelia is a great gift. Hats, bags, wallats, games act...
Oh, and be prepared. When Kenyans marry you can bet the whole village to show up...
I'll get back to you on the cows...
Congrats to you!!