Retarded argument.  "someone has done $600 dollars worth of work".  Are you assuming that it ALWAYS took $600 of work to produce these coins?  What if bitcoin price drops to $300 the next day, are you going to change your argument to "someone has done $300 dollars worth of work"?  How is that "backing" exactly?  Do you even know how many bitcoins were produced when the production cost was only $10, $50, and $100 (hint, a whole lot more than when the BTCs that were produced at $600)? 

Your argument would be partially correct if bitcoin was a PERISHABLE consumable commodity, where the end buyer chooses to consume the commodity or modify it into some other kind of product, such as coffee, corn, etc.  And the original commodity is used up in the process of production (i.e. no longer exists).  If this is the case, the original producer of the commodity will always take the production cost into consideration.  And I stress perishable because the commodity has a very short lifespan if not consumed, hence the commodity price *SHOULD* closely follow the production cost because the production cost happened just *recently*.  Unlike your flawed argument that bitcoin "cost $600 to produced", which is obviously not true for coins made 2 years ago, last year, or even 6 months ago. 

And I said *partially* correct.  This is because even in the case of these perishable consumable commodities, the price is still mostly driven by supply/demand.  Think about it.  If the market demand stays constant for corn, and there is a GREAT harvest for corn.  Corn supply will greatly exceed demand, and the price will drop.   This is where the importance of *perishable* part comes in.  If the farmer cannot sell the corn in time, it will go bad, hence he has to reduce the price even if it meant that the production cost exceeds the prices he sells at (i.e. selling at a loss). 

So the lesson for you today is that production cost is only a small factor in determining market price, even for perishable consumable commodities (which BTC is obviously not, since once it's mined, it's mined, it does not expire.)  Everything is driven by supply/demand.  So there is no backing from production costs whatsoever.  Furthermore, you don't even bother to analyze what is the average cost to produce all bitcoins, instead of the current cost of production, which makes your entire argument fall apart in 2 seconds (i'm just really sad no one else pointed this out, tells you the average IQ of people on this forum).