What's the Budget for Writing a Textbook?

To me, the key line in Free digital texts begin to challenge costly college textbooks in California:

Yet he turned down $100,000 to turn over his open-source textbook "Introduction to Economic Analysis" to a commercial publisher.

I know nothing about what professors get paid to write college texts, how the deals are structured, etc. Let's assume that $100,000 is a reasonable offer.

The point is not that in the future many other people will pass up $100,000 to write a textbook. The point is that there are lots of actors other than publishers who can pay a professor to write a text and release it under a free license, and $100,000 is not a lot of money to a state, country, large university or foundation.

I also found this quote from Bruce Hildebrand, the Assn. of American Publishers' executive director for higher education, to be amusing:

God bless anybody who has got the energy and commitment to put three, four, five years of labor in on a book and then give it away.

If that's what it takes to write a textbook, I'd want more than $100,000.