In litigation, there is the expectation that parties shouldn't be expected to bear discovery costs that are out of proportion with the value and relevance of the requested data. "It'll cost too much" is thus a stock answer to discovery requests. Google can absorb the $100k cost--they'll likely spend tens of thousands to just to litigate over the discovery dispute. But they're entitled to hold the government to it's burden of articulating why that data justifies the cost.

Obviously the real reason is that Google doesn't want to hand over the data. But you should never just hand data over to the government. Government lawyers can get pretty creative with spreadsheets of numbers.