I really respect people who offer a clear free value ( not crippled ) and the paid version is a no-brainer. This is easy to demonstrate when what they're paying for clearly takes more resources to provide and support. A 'free' resource should be enough for a person to try and make sure it works for their use case -- but shouldn't be sufficient as a business resource for a profitable company.

Also remember, that the more you earn, the more you can spend to get customers and keep them happy.