It is very difficult to get a perfect answer to this and we are only really able to give a guesstimate.
The software device has an estimator built in to it.
The hardware device requires some hand calculation:
Size of each transmission = header + (20*number of channels) (in bytes)
(Where header = 120 000 byte)
You can then work out how many transmissions per month and therefore the amount of data that will be used (very roughly!)?
So, for 2 channels at 1 update per minute we would have
transmission size = 120000 + 20*2 = 120040
Number of transmissions per month = 60 * 24 * 30 = 43200
Total data = 120040 * 43200 = 5.12 GB
There is very little relative effect on the total data by increasing the number of channels.
Please also remember that this does not allow for the extra communications involved in recovering lost connections and multiple extra status messages.