WARNING: If you sign up for a cloud mining service, you will lose most of your money as explained in detail here.

As mathematically proven in the above link, if you buy a 15Th/s contract and with a 5% difficulty increase each round, you would only receive 45% of the bitcoin from mining compared to just buying bitcoin directly. And that's just with a 5% difficulty increase per round. Over the last 3 months, the bitcoin difficulty increase has been around 11% on average so you would receive even less than 45%. Also, given the way the price of bitcoin is rising, you can expect even greater difficulty increases further reducing the amount you would mine.