I don't think a preacher being financially successful outside of the church is the problem. I think when a preacher is doing better then even a single member of his congregation and his money is coming from the congregation that's when it becomes morally questionable.
Being successful outside of the church is not the problem (as long as the preacher doesnt ask the congregation to give more than he would) or accepting help from the church, but to be doing better than his congregation off of their sacrifices seems wrong to me. That's where the problem is in my opinion. Jesus accepted help but I'm sure he wasn't living better then the people that he was accepting help from.
Again being successful is not the problem it's being successful off of others generocity that is. If you were living better than your congregation then there is a moral problem even if it's just a SINGLE member of your congregation that you're doing better then because that would mean you're accepting help from someone you should be helping.
Regardless of morality though in the end it seems both possible and acceptable to make a living off of a christian church here in America. Preachers from all over this country do, everyday. It is an option and a choice that is made by someone for whatever reason. And they are able to live off of this choice whether it be wrong or not.