I'd pay. I've tried going the Google Adwords route with one of my sites, and the return on investment was poor (definitely greater than $1 per sign-up), while waiting for organic traffic to come from Google despite all the back-links and SEO you can think is like waiting for your hair to go grey.

$1 per user (including them actually engaging with your site: adding a profile, posting some basic content etc.) sounds like good value to kick-start a new project.