I'm really surprised to see such calculations from a Legendary person. I believe you just misleading newbies, because you should be well aware that the statement
buying a contract you thing the bitcoin price will rise the same or more as the bitcoin mining difficulty.
is wrong. If you make such assumption, it may happen (and likely will) that holding bitcoin would be more profitable than mining it.
If you look at the most recent development, ZetaOS is correct - about the last year bitcoin price growth was bigger then (or equal to) the difficulty. But in the past years the opposite was truth: difficulty was rising faster then bitcoin price. So how this will be in the near future can't be foreseen.