Tamaki's reasoning for making Haruhi a host?
The way I see it, Tamaki doesn't care about money at all. He started the host club merely as a form of entertainment for himself. When he noticed how good Haruhi looked (even in "male" form) he decided that adding "him" to the club would only make things even more interesting. While I have found nothing to actually support any of this, it's something that I believe is a very plausible answer.