Some suggestions... if you can make "iChat for public" become very annoying like PalTalk's free version with all the pop-up banners, people will pay $20US for it a year.
I think the free trial idea is better than the 15 min chat per post idea. The "flow" is the issue, you want to keep talking and talking to understand the advantage of chatting in realtime.
If you can make it easy for people to start a chatroom, and be a chatmaster... then consider the price of getting the chatroom running from Digichat, maybe iHub can resell the each chatroom at a very cheap price per month. It just cost too much to start one with Digichat.
The individual stock chatroom should be free, much like the free IH message boards for stocks.
or... look into PalTalk and see if there is a business version of PalTalk to integrate the voice and text chat in IH.
JMHO
signed,
Bernard