My experience with Hologram Pilot Plan on the 2g network was as follows:
3 parameters (Conductivity, Temp, Depth) returned in addition to DateTime @ 15 minute intervals
Average daily use = 715 B per day. As an example we used 1.34 MB in the month of March 2020.
So for 1 station the cost is easily < $20 per year (in the pilot plan). However, like Shannon said 2g will be going away at some point soon, depending where you are, so cost for 4g may be different. I do plan on updating to 4G at some point.
Hope that helps!