The American Osteopathic Association and American Optometrist Association are pretty well set with their urls, but there are bound to be companies like:
http://www.aoa-gps.com/
that either don't have any domain name or would like to upgrade.
Hard for me to say what $$$ a motivated buyer would be willing to pay... but some folks here have more experience with three-letter .COMs and may venture a guess. I think it is a useful name.