Probably in the millions.
You're talking extra wages for people that are paid and not on salary. You're talking extra fees for all of the gear on site - if you want them available for an extra day "just in case" you're paying something to reserve it for another day. If you're building that extra day into the contract of the people and companies providing and using that gear, they build that cost into their quote and the overall cost of the event goes up just for the added contingency of needing to rain delay an event by a day - something that might happen every 10-20 years or so.
There's the logistical cost of keeping everything on site for an extra day and shortening the turnaround to the next event by a day. That's a lot of people working extra hours because the already tight turnaround of moving the team's cars, garages, pit wall booths, communications, VIP and corporate lounges, and probably a bunch of other things that I'm not thinking of, has been shortened by a day. Losing a day in production like this is massive.
I've already lost my train of thought around all the things involved in an event like this but trust me, when the word expensive is thrown around in this world, it's insanely expensive. I've seen NFL teams throw around tens of thousands of dollars without blinking an eye. Something like this gets them blinking both eyes rapidly.