Further investigation confirms the large amounts are for real. Lots of considerations if one decides to sell all or part of their holdings.
Pretty much a given that the amounts being offered in the mailings are lower as it’s just one buyer. Working with brokers, such as mentioned in this thread already or with outfits like the US Mineral Exchange, exposes the rights to multiple possible buyers depending on the broker and their contacts. Fees through brokers for the sale can run anywhere from 6 to 10% depending on the broker.
The low ball offers are just fishing for the one in a thousand who foolishly bites, the profit for the buyer is huge and makes it well worth sending out thousands of letters.