Think of it just like a gold mine. First you want to make sure there is more gold in the mine than what you paid. If the price of gold is $1400 an ounce, and you paid $140,000 for the mine, it had better produce at least 100 ounces of gold. So you retain a geologist to estimate how many ounces of gold are in the mine. If it is less than 100 ounces then you should not buy the mine. Now you can keep digging, but after you mine out the rich veins (low difficulty) you will get less and less out of a days work. After a while, it will cost you more to dig than you get back from selling any gold you manage to find.

Some people might say "but gold will rise in price and you will make money that way!". If that were true, then you should just buy gold. That way you don't have to bother digging for it, you know exactly how much you will have, and you will pay a cheaper price for it than overpaying for a gold mine.