You wouldn't parse the watch command, you'd parse whatever command watch was running. I pimped your search to match a wider range of possible output from ifconfig:
$ ifconfig eth0 | grep -Eo '[0-9\.]+ [PTGMK]i?B'
This is made possible by grep's
-E argument which allows a wider syntax and
-o which only outputs matching strings.
If you want to loop that, you can but you have to wrap it in a shell so the pipe is interpreted correctly:
watch -x sh -c "ifconfig eth0 | grep -Eo '[0-9\.]+ [PTGMK]i?B'"
But in my opinion, this isn't wildly useful as it is...
watch is really only good for a real person watching the screen. If that's the case, you're all done but if you want to do something with these numbers on a regular basis, you're probably using the wrong tool.