The amount of data used by a network radio device varies with use.
It also very much depends on which applications You use on the device.
There are many different applications available for network radios and mobile telephones that provide 2-way simplex communications functionality, some applications use more than others with the 'Zello PTT' application being one of the best due to using an advanced rate adaptive/variable bandwidth CODEC (coder/decoder) that compresses the audio efficiently while providing very good quality audio.
A Sim card deal that provides 200mb of data per month should be more than enough for occasional users who have a single channel switched on at a time and have 'sending and receiving images' turned off.
1 GB per month should be more than enough for the majority of people using Zello every day with image sending/receiving enabled.
With multiple busy channels in use for long periods of time You will require a larger data package.
I personally use multiple Network Radio devices every day, these are often connected to the networks and operational for several days at a time. Using the Zello PTT application on One channel at a time (other channels switched off when not in use) has used less than 500mb per month .
If You have multiple channels switched on all the time, especially if they are busy channels You will use larger amounts of data.
With 12 channels enabled all the time, most of which are very busy, I have been using between 1.5Gb and 2.5Gb per month.
(This total includes sending and receiving images on multiple channels, occasional web browsing, checking email, uploading photographs and short time-lapse videos + using GPS applications that access map data online)
You will find that other applications such as web browsers, GPS tracking apps that download map data, and full duplex communications applications such as Skype will use much more data than simplex PTT voice communications applications.
Most network radios can also be used as a WiFi Hotspot allowing other devices to access the internet through the network radios 3G/4G data connection, this will require a larger 'data allowance' with Your chosen mobile network provider.
Video streaming applications tend to be the heaviest data users so if You intend to watch video or stream video out onto the internet with Your network radio device (or other devices tethered to the network radio) You will need a much larger data package deal.
Of course if You only use Your network radio on home/office/work WiFi connections or tethered to a mobile hotspot on Your phone, portable 4G wifi router or other mobile internet wifi hotspot device You will not need a mobile data SIM at all.