Simple multiplication.  It is what 27 PH/s would cost at the various price points offered by various vendors. (i.e assumming they could deliver today buying 27 PH/s from BFL would cost you ~$1.35 billion).  In order for the hashrate to rise to 27 PH/s that means someone had to buy 27 PH/s of gear.  At the current hardware prices and exchange rates I don't think that is likely.

I guess another way to look at it is how much hashing power (in USD) do you think has been bought.  My guess (SWAG) is that it isn't enough to push us to 27 PH/s as fast as you think. Note this doens't mean buying more hardware is a good idea certainly not at current prices but I think the law of large numbers is going to slow the rate of hashing power growth.