Here's the maths for Genesis Mining . It may be a couple of week old but that doesn't invalidate the point.
Basically, at 2500 USD/BTC. Genesis Mining would have cost 0.78 BTC for 15 Th/s. With a 5% difficulty increase each round, you'd only mine 0.3539 BTC before the contract became unprofitable (you would have to pay GM money to keep the contract rather than make any money from it). That's a mere 45% of the cost of the contract after about 310 days of mining.
Obviously, if you just bought BTC you would have had 0.78 BTC instead of 0.3539 BTC. At 2500 USD/BTC, 0.78 BTC = 1950 and 0.3539 BTC = 885 USD. AT 3800 BTC/USD, 0.78 BTC = 2964 USD and 0.3539 BTC = 1344.82. The fact that BTC price has increased is irrelevant, you still have twice as much BTC and it would stil lbe worth twice as much USD.
You're right that cloud mining can be less costly than mining yourself for some people but if that's the case you shouldn't be mining at all for financial rewards as you're just going to loses money. Some people do it anyway for other reason such as helping to decentralise the network. Those reasons don't apply to cloud mining though.
The only way to earn money from cloud mining is through affiliate links which means you're ripping other people off to make up for your loss. In other words by scamming other people.