Not sure how big the headers for ntrip are, but the corrections themselves are very small.
I ran RTCM3.2 through RTKlib server and it was reporting just under 4.5kb/s for all constellations at 1hz, 25 sats visible.
So that would be 370Mb in a 24hr period, or 15.5MB/hr. (1024kb/mb)
Some data savings could be achieved by not using all constellations at the base. As it will send the corrections anyway if selected on the base, even if the rover is not using them.