May we kindly ask if it is possible to obtain a China visa in Hong Kong for an American national?

It may be possible to apply through some local tour agency, otherwise, you cannot get it.

The quickest time I heard is one day. You can get it within 24 hours. Generally speaking, you need to provide your hotel booking in China, round trip tickets to China, your valid passport and application form and your proof of legal status in HK.

For the fee issue, you may need to consult at the tour agency directly, because different agency charges a little bit different.