I frequently use my Macbook on my iPad's data plan. Unfortunately, unlike iOS, OSX can't limit application network usage based on whether it's on a cell network, and I always have to remember to close Steam, uTorrent, Dropbox, automatic updates, etc. whenever I start tethering. Is there an app out there that I can turn on and off quickly to whitelist a small number of applications and prevent everything else from using the network?
TripMode was made specifically for this purpose, and even turns on automatically when connected to designated cell hotspots.
It looks like Little Snitch allows you to configure whitelists, blacklists, and even more advanced configurations for network access. You can make a cellular profile and switch to it when you use your tethering. It also measures how much bandwidth you've used, per app and total. You can even automatically switch profiles when you connect to your tethered Wi-Fi or Bluetooth network.