A good investiment in my opinion would be casino bankroll. If you invest $20.000 on it (28 BTCs), you can earn 4.48 BTC monthly income... I don't know if the casino limits a max investiment allowed. And it's not guaranteed, I'm considering you can make 16% profit monthly, but it surely can change. Tell what do you think.
About what gambling site you are talking ? Earning monthly 16% in a passive way must be very good amount, very much eager to know the name of gambling site that you are mentioning here.
But I too feel definitely there would be some upper limit for the maximum investment an investor may put in. Because, practically a gambling site might need some investments definitely will not be unlimited investments.