Paying $0.001-$.01 to bypass a paywall is arguably a very low barrier. Allowance for children, poor people, etc would be needed, of course. Maybe amusing image-mapping captchas would be workable. That could even be an income stream, Mechanical Turk style.
That may be true for countries with online payments that actually work, but keep in mind that for many third-world countries the jump from $0 to $0.001 is hard, since people often do not have any way to perform an online payment.
Paywalls also have a negative effect in the linkability of the Web. If I suggest you to go to some page, and you have to pay for it, then independently if the price, I have a much higher responsibility for the content actually being relevant for you.
The cost is just not money, but also time. It takes some time to fill in payment information, even if only a minute, and even if only once per website.
