How do I profit from a Plugin?

The way I see it you have two options

Paid for plugin

The idea of charging for the Pro version as you describe is a good one. We use a couple of paid for plugins. They tend to be reasonably priced when compared to the cost of setting up and running a website and work well. It’s also nice to know we can customise the plugin if needs be as the files are on server (although I am not sure if this is “good practice”).

Software As A Service – SAAS

The other option would be to actually offer it as SAAS, ie as a hosted service based around WordPress using this plugin. I am thinking of your standard “sign up for XXX dollars” and you receive a hosted version of WordPress. I suppose a little like WordPress.com. As long you target audience aren’t other developers, they will love the idea of not needing to install or manage anything and pay extra for this.