Hard to say. So far, I've only given out my phone number to online services like gmail (woo 2 factor authentication!) or banks, but that's because my email and bank accounts are more powerful than my phone number and because very few services ask for it. I think there's a chance I wouldn't give out my phone number, and I can't clearly feel whether that chance is larger or smaller than my reluctance to pay $5. (Modeling myself from over a year ago is tough.)

This also runs into the trouble that instead of getting resources from users, you're spending them on users- texting activation codes is cheap but not free.