The only thing that has the potential to ruin such a business is Jagex removing the free trade.
I see it highly unlikely they would remove it.
But take into account, if you get 1 player a week wanting to become a host and paying 6B as well as the countless amounts of money they take in on 10% of bets, $40-80 I don't see as seriously outrageous.
If each host makes 500M a day, and you take a 100M commission. call it 1-2B a day on commissions, that's 7-14B a week + 6B on new host making a potential 20B a week selling @ $.5 for instance is a not so unrealistic $10,000 a week.