It sounds like you're talking about the WAN data usage (for if your ISP caps your data usage for example).
If that's the case, you can find some data under "Network Wide" -> "Traffic Analysis" and select traffic analytics for your security appliances.
Then next to the work "Usage", you'll see the total amount down and up.
I'm not sure whether this accounts for SD-WAN tunnelled traffic.
Another interesting yet less elegant way is making a custom pie chart to track internet bound traffic vs internal traffic.
It's not elegant but it may work for you.
Custom Pie Charts in Meraki – dunxd.com