defintion found on this forum:
Post-Institutionalized Child: Children adopted from institutional, hospital, or orphanage settings. The term is used to describe an array of emotional and psychological disturbances, developmental delays, learning disabilities, and/or medical problems resulting, in part, from their stay in institutions.
I don't think there are any statistics on how many children are in foster care vs. SWI's. It does cost more to be in foster care and there are several US non-profits that are funding foster care (Half the Sky comes to mind).
And here's a resource:
http://www.attach-china.org/