Using airline miles?

My wife has about 85,000 American and I have about 35,000 American airline miles. We are planning a cruise vacation in South America and would like to use airline miles to cover the airfare for one of the tickets. For our flight times, it will cost 100,000 airline miles. So, I am thinking about giving 15,000 of my miles to my wife so that we get one "free" airfare. How much does it cost to transfer airline miles? Am thinking this should work and really offset the cost of this trip.