measuring data exchange over external network
You can see the total number of bytes transferred per network interface using:

netstat -ib


or for an individual network interface (e.g., en1) using:

netstat -b -I en1


You can monitor the amount of data per program using:

nettop -P -t external


The "-t external" includes only external traffic.

You may be interested in the amount of traffic per web page.  If you are using Safari, then select >Develop >Open Web Inspector, and select the "Network" tab.  This will show which files are transferred and the total size of the web site.  Unfortunately, it counts cached files in the page size, so it does not truly reflect the amount of data transferred.  Firefox and Chrome have similar tools.