Why not ask you friends and colleagues who have an Android phone? Show them a mockup, what the app will do, ask them for feedback and wether they would pay $xx

Why not ask you friends and colleagues who have an Android phone? Show them a mockup, what the app will do, ask them for feedback and wether they would pay $xx for it. Watch them cosely and only count those who are competely into it, divide that by the total number of people who asked and you have a rough percentage that you can apply on the number of android users in the U.S or elsewhere.