Before you can reduce the amount of data you use you will need to determine just what devices, programs and processes are using your data.
On Windows based computers I suggest you download and install GlassWire. The free version is fine.
Understanding the output:
Most users are really surprised at what is running as background processes on their computer.
It is important to understand that Glasswire will only log and report data usage on the computer upon which it is installed. If you have multiple Windows computers you will need to install Glasswire on each and correlate the usage data.
Glasswire will NOT be able to detect and report on router based security issues/leaks and that is a subject that we absolutely need to review.
At the heart of your network is your router. Every data byte you and your devices consume have to go through that central point of your network.
I would very strongly suggest that you review the capabilities of your present router to see if it has a "Traffic Analyzer" function.
I also strongly suggest that you review your routers internal settings in regards to:
>All guest accounts disabled<
>Routers GUI username and password changed from default<
>All wireless frequencies have strong encryption enabled<
>Remote Access function disabled<
>All internal router "cloud & sync" functions disabled<
>All internal router "services" be reviewed for data usage<
If your present router does not allow track data usage per device I would strongly suggest getting a router that does.
I currently run an Asus RT-AC3100
Statistics per device:
If you have a complex network made of wired and a mix of wireless devices it is only at the router level that you will have a complete picture of just what devices are using data and what programs and just as importantly what background processes are consuming data.
The Asus AC-3100 pictured above may be a little pricey ... there are less expensive Asus models that can run the Merlin firmware and has the Traffic Analyzer function. Some models can also be found as full warranty factory re-furbs..
Before you can address you data "burn rate" you need to find out just what is running on your network.
Generating a $300 monthly bill is indicative of massive data use more in line of a user with a terrestrial connection rather than a data capped satellite service.
While cancelation is available to you so is doing a careful review of what in particular is using your data at such a terrific rate.
There has been a number of substantial changes in Operating Systems, OS Update delivery, hardware driver changes from Nvidia that results in massive data use in the form of "telemetry".
To that we can add the potential of unsecured aspects of your Network.
You may be surprised as to how much data is being "wasted" by unknown/unseen programs and processes..