idle traffic considerations

[Knuth]

Hey,

we are planning to deploy certain devices with an embedded sim cards in 
different countries across the globe, for maintenance we need to be able 
to connect to the devices with ssh.

Since the sim cards only provide us with a private IPv4 behind NAT 
(because apparently IPv6 is still hard...) we need to reverse the 
connection process to our control system,
at the moment we consider doing this with wireguard (we are aware of the 
"pre" release status), since we had good experiences with it on other 
similar setups.

To calculate some rough estimated costs for the mobile connection 
traffic volume, i'd love to know if there is a way to calculate the 
amount of traffic caused by an idle wireguard connection kept alive 
since we would be charged per MByte transferred.
Or do we simply have to setup a few test subjects and monitor it over a 
longer time, which in itself could be error prone.


Thanks for your time
Knuth