I don't understand where the cost comes from. Can't you double-spend at will until one of your transactions makes it into the blockchain? Until that point, as far as the Bitcoin network is concerned, you haven't actually spent any money. So it seems like you'd have ten minutes or so of double-spending ability, totally free, and only have to start using processing power if you want to extend beyond that ten-minute period (by forking a blockchain that excludes the transaction you intend to double-spend).

My understanding of Bitcoin implementations is pretty vague, though, so I'd welcome correction if this isn't actually how it works.