It’s an interesting idea to try putting an order of magnitude cost on the system.
The total “netspeed” for the US zone is 86798173 (“kbps”), so one 1 gbps system should get a little over 1% of the US SNTP traffic (and the same amount of new NTP traffic).
If the cost for all that traffic was similar to what you saw, that’s about $64,000 per year just for the US.