> i said in my etsy/san telmo post the other day that more tractors are sold every day in farmville than are sold in the US every year

The supply of real tractors is limited by a large number of things that cost real money including raw materials, labor, and so on. The supply of virtual tractors is not really limited by anything (marginal cost is pretty much 0), so the cost ought to be 0 or awfully close. Granted, there are costs to 'develop' virtual tractors, but those don't limit the production of them once they've been created.