http://blog.pinboard.in/2011/12/don_t_be_a_free_user/
While conducting the beta test for my selective link sharing-via-email digests tool, Handpick:
I asked my users whether they'd pay to support the app to run for the long haul. They love my app enough to say yes and were delighted that they could actually actively fund the operations and development.
Go on, try asking your users if they'd pay for your product. It's the best validation you can get!