Since nobody has pointed out the obvious: buying things for $1 in the business world means that the thing has negative book value (= its liabilities exclude its assets), but not so much that you'd want to structure the sale the other way around (= Provo paying Google to get rid of the thing). So Google's not getting $39m worth of stuff for $1, they're getting $39m worth of assets plus $Nm worth of liabilities (where n > 39) for $1. See also: http://en.wikipedia.org/wiki/Peppercorn_(legal)

Also, paying $39m to build something doesn't mean the resulting asset is actually worth $39m any more, but that's another story.

(And obligatory disclaimer: I have no personal knowledge of how the valuation was actually made.)