And, while you're at it, please cite a scriptural source for Buddha's insistence that all Buddhist books be free. (Nevermind: you won't find one).
It is up to the individual to determine whether Dharma has value, and what that value may be. In our current Samsara, money is a primary measure of value, and is traded for other things one values.
At the time of Khyungpo Naljor, and Marpa, gold and textiles, among other things, were valued, and prospective students offered these things to teachers for the transmissions they received. Marpa actually had to return to Tibet to make money so he could go back to India for more teachings.
Although Buddha surely wished for the enlightenment of all sentient beings, I am certain he did not wish to equate the value of his dharma with "no cost."
Frankly, there are many who obtain a bunch of books, or obtain copies of material on-line, freely, via various means, and they value these materials about as much as they paid for them.
In the grand scheme of things, of course, one doesn't REALLY need books. But I think we all understand that.
"Absolute Truth is not an object of analytical discourse or great discriminating wisdom,
It is realized through the blessing grace of the Guru and fortunate Karmic potential.
Like this, mistaken ideas of discriminating wisdom are clarified."
- (Kyabje Bokar Rinpoche, from his summary of "The Ocean of Definitive Meaning")