While I remain committed to the open source foundation of WordPress, I do prefer to make a living plying my trade as a developer of quality software that fulfils a need of people willing to pay for it.

I have developed a plugin for use by mainly business users of WordPress, i.e. cases of people using blogs to market their business, or trying to use WordPress as a CMS as well. I would like to 'sell' a 'pro' version this plugin, while still having a base version freely available. I would also like to avoid asking someone to pay for something they don't feel they have to, like charging a bloated distribution fee or something. 

I am considering that users of the pro version must be registered members, with a registration fee, and an online registration check when starting the plugin. What other options are available here?

**EDIT:** 

Inspired by Zweiblumen's mention of SAAS, I have envisaged a model where the freely available, no registration, code is more of a skeleton which access the business logic and data through a web service. What do readers think of that?