I'm curious about this as well, but thus far haven't come up with much hard data on the topic. Just thinking about the families I happen to know, I'd venture a guess of around 20-25%. Higher than the general population, for sure, but far from guaranteed.
My family: I have a birth son who is NT with a few potentially AS characteristics, daughter is completely NT, both with an NT biological father. Mordok and I's son is autistic. (skewing the sample there though because we're both AS).