If your chat page is behind some sort of authentication, the easiest solution might just be an open source script like Ajax Chat from Sourceforge, however as diafol suggests, you'd have to clarify your requirements. Things like once the user is logged in, their username should auto-log them into the chatroom when they come to that page must be considered.
By any chance are you using WP? If so, there are already numerous plugins that are free and already built for it.
Had a look at Rumbletalk. Seems OK, nothing special, but they want $14/month just for a chat box! Having used the demo, I wasn't too impressed.
c-box has a free version, but the number of missing features doesn't make it very attractive. The premium version is only $2/month. It looks pretty rough though. I'm not sure why those two were suggested to you.