In the grand scheme of things, not a lot of businesses use Apps for their email, but I'd hazard a guess that you're still looking at hundreds of millions of dollars in losses if that were to suddenly disappear.
It's the cascading service failures and related economic costs that would be the most damaging (and hardest to really quantify until a scenario really happens).
Wow, I had no idea that Google Apps were about 15% of Google's revenue. That is a way bigger than I would have guessed. (Hundreds of millions x 2 orders of magnitude is >= 10B, total revenue in 2014, 66B)