Look at cex.io right now. You can buy 1 GH/s for 0.2125 BTC, but 1 GH/s will never mine more than 0.1850 BTC (assuming an average difficulty increase of 20%).

Even assuming that cex.io is legitimate, why would anyone pay 0.2125 BTC to mine 0.1850 BTC?

I don't think this business model will ever work simply because there is no reason for cex.io to mine bitcoins for someone else when they could make more money by mining for themselves.