I'd be glad to pay now that I've enjoyed reading it… but I'd almost certainly not have read it if I had had to pay first.

I like the approach taken (among others) by Matthew Butterick for Practical Typography [1]: I've enjoyed reading his book the same way and gladly paid for it /after/, as he asks, even though I would not have bought it at first.

[1] https://practicaltypography.com/