I am not entirely sure, to be completely honest. In my experience, it is very little but it varies too. It really depends on how many people connect, for how long they connect, etc. If you have limited upload speeds, maybe it wouldn't be a great idea to run it in your browser/phone. Maybe try running it directly on your computer using the -capacity flag?
I haven't been able to find any specific numbers either, but I did find a post on the Tor Forum dated April 2023 or a user complaining about high bandwidth usage. This is not the norm in my experience, though.