I skimmed the methodology, my understanding is that the estimate comes from taking 16% of annual booster support for each school:
https://nil-ncaa.com/methodology/
They get the annual booster support figure from this report:
Specifically, from the “Contributions” category, which is defined as:
Contributions: Includes amounts received directly from individuals, corporations, associations, foundations, clubs or other organizations for the operations of the athletics program. Amounts paid in excess of a ticket’s value. Contributions include cash, marketable securities and in-kind contributions such as dealer-provided cars for staff use. Also includes revenue from preferential seating.
So our most recent data reported $62 mil in contributions, which was a big jump from the $24 mil it was before (also for some reason 2021 and 2022 have the exact same figures):
16% of that doesn’t quite add up to the number in the NIL table, so I think they’re doing some sort of backfill calculation that gets them to the total NIL number for P5 schools that is on Opendorse.
TL;DR: lots of assumptions, not a real reported NIL number for each school.