He could if people were really interested in the money. People, however, are more interested in keeping his check and autograph as a collector's item and/or bragging rights.
It's hard to explain, but if I had the choice between a $100 textbook with Knuth's signature, or a $2.56 check for finding a bug, I'd take the cheque. It's such a unique way to thank people, unlike an author-signed book.
