Webfont cost
Type a Google Fonts family and this page downloads the WOFF2 files it would serve, then counts them. The Arabic file is sometimes the heaviest, sometimes not: Cairo's is smaller than its Latin one. Either way nobody knows until it is measured.
Nothing measured yet.
What this measures
Bytes on the wire for the WOFF2 files Google serves for that family and those weights, counted in this tab. Nothing is estimated and no number is cached from another visit.
What it does not
It does not measure your server's compression, your cache headers, or whether the font is used at all. It does not judge whether the font is any good, whether the joins are right, or whether the marks sit where they should. Those need eyes.
Google splits every family into unicode-range subsets. A visitor downloads only the subsets the page's characters fall into, which is why the Arabic number matters more than the total: an Arabic page usually loads the Arabic subset and the Latin one, and skips Cyrillic, Greek and Vietnamese entirely.