[crc32]

So... I pull water out of my well at a cost of $1 per bottle, later sell for $5 per bottle, have $400 in my hand, and yet my profit margin is zero.

I am not sure how to respond to this. You are self-evidently wrong, unless we are to accept your redefining of profit to be "the selling price of goods minus the selling price". Very strange.

[sebg]

"I pull water out of my well at a cost of $1 per bottle." This is the same as buying a bottle of water from a supplier. Your input costs $1 per bottle. You sell it for $1 dollar. Your profit is zero and your margin is zero. (profit = revenue - cost of inputs // profit margin = profit / revenue )

"I later sell if for $5 dollars per bottle." If you were to buy from a supplier now because there is no ferry it would cost you $5 dollars per bottle. You then sell the water for $5 per bottle. Your profit is zero and your profit margin is zero. (profit = revenue - cost of inputs // profit margin = profit / revenue )

So your profit margin is still zero.

You have $400 dollars in your pocket though. Where does this come from - this comes from speculation. You "invest" in 100 bottles of water at $1 dollar. The market for water goes up so people are now willing to buy bottles of water at $5 dollars. You speculation profit = revenue - cost of inputs[where this is the prior cost for you]. The speculation profit is $400 dollars.

This is the answer to your other comment [ http://news.ycombinator.com/item?id=1118301 ].

These are two types of profit.

profit = revenue - cost (at the same time)

profit margin = profit / revenue

speculation profit = revenue - costs from an earlier time

[crc32]

You are wrong and I think it is important that you understand your error. Your mistake is that pulling the water out of a well that I own is not "the same as buying a bottle of water from a supplier".

I have pulled the water out of a well that I own, at a cost of $1 per bottle. The cost of production is constant, over the time period that the ferry is both available and unavailable. Therefore should the price that I am able to sell my water at rise due to constrained supply, I will make more profit. My profit margin will increase.

This is called "supply and demand", you will find it explained on Wikipedia, and it is a basic concept underlying markets of all kinds.

What I meant by "there is only one type of profit" is that profit in all your definitions can also be stated as selling price minus cost price. In the context of this discussion the distinction is meaningless, because regardless of how you obtained that margin, whether by speculation or not, it does not neccessarily tend to zero in open markets!