A two constellation correction (just GPS and GLONAS) is only about 1gb a month is used constantly. If you use all 4 constellations then it can be over 2gb. Not many people running a tractor 24 7 for a whole month though.
So far a £7 the month sim in my phone has been working for me. I do have an EE mast in the middle of the farm, so no need for a SIM that works on all networks and I'm using using my own f9p base and rovers. The commercial ag systems may vary.