Its not cheap. I think testing just for prothrombin is $500 or more, can't remember. There are a bunch of clotting disorders. Once you test for them all, then you just have to test your kids for what you have. I had both of my daughters and parents tested for it. My mother has it, so she gave it to me. One of my daughters got it from me, so that's right along with the probability of 50% as predicted.
Oh, with my insurance, I think it paid for the testing on my family because I had already been diagnosed with it.