Just to really throw some confusion into the mix, I’ve got a site with a little over 750 hosts (a little under 30k services, how about you?) pulling an average of only 8 Mbit/s 
It’s going to vary a lot by what your environment looks like. Some service plugins return LOADS of details, simple systems won’t have much to say. You can always cut the data rate in half by doubling your check interval too 
But yes, I would also advise running the monitoring site in the same… zone(? whatever the Google Cloud term is) so you’re not paying dollars on the monitoring traffic. If you’ve gotta pay for traffic between zones, go for another distributed site in the extra zones. The enterprise license lets you scatter sites pretty freely, it’s just the total amount of monitored stuff you pay for, regardless of the deployment structure.
Wish I could give you an exact figure on what the traffic of interacting with a site actually costs you, but I don’t have an interface monitored where that’s the only thing. The daytime traffic looks to be… maybe 300kbit/s above baseline average, if I had to pick a high estimate? Peanuts.