This comes from Mike on Tusi 2 - one of several boats that discovered the high internet costs in French Polynesia.
It gives an explanation and offers a partial solution.

In French Polynesia the cost of internet is very expensive NZ$70/GB. However, automatic app/software updates can chew through data at an incredible rate. Before I figured out what was happening I watched 400MB (2000XPF or about NZ$28) disappear in 5 seconds. I loaded another 400MB pre-paid card and that 400MB went in 8 seconds! I can see the timing on the software in the wifi hotspot we bought.

The partial solution to this in Windows 10 is to go into Properties
for the wifi site and make the site a metered connection. This is in
the pc setup. This doesn't completely eliminate uncontrolled updates
because Microsoft will still do updates over a metered connection if
they decide that the update "is essential for the continued operation
of the system". Google seems to have a similar policy for their apps.
However it certainly helps a lot. I've found that if I haven't used
the internet for a while like when we were in the Tuamotus then when I
connect to the internet again, usually back in Papeete, quite a bit of
data vanishes as these "essential" updates are downloaded. But once
that's done date usage is pretty reasonable at least for what we do.
Working on a website is probably a very different story.

I seem to recall that if you are still using Windows 7 it is
possible to stop all updates. If that is the case then that would
certainly help.

On a phone a metered connection is set up in the Settings under "data
usage". At the bottom of the that page there is a heading "Network
restrictions". In there these restrictions ie automatic updates, can
be turned on for any wifi site.

So with this all done on our phones and pcs, data usage isn't too bad but there is no getting around the fact that it is expensive.