Are you considering to sell premium WordPress plugins? Here are some existing business models.
Easy Digital Downloads provides basic support for free and offers priority support for monthly and yearly price.
A very popular way to get widespread usage for a premium plugin is to give it free of cost while charging for extensions.
This is a very sound business model if your plugin is extendable. This also provides an opportunity for third party developers to contribute to the plugin ecosystem. In some cases, the cost of essential plugins for a project might cost more than usual premium plugins. Some plugin makers are also criticized for offering only a bare bones version for free while leaving essential functions to be bought as extensions. But they defend by saying that this is the only way by which they could keep the plugin actively developed and make a profit out of the business. Fair, huh?
Woocommerce, Jigoshop, Easy Digital Downloads and Yoast follow this model.
Sell yearly licenses and limit usage by different packs.
This is a very common business model which provides the plugin for an affordable cost to individual users but charges a premium for use in additional or unlimited sites by Developers.
Backupbuddy follows this model.
Plugins from iThemes – I think iThemes is the only plugin company offering a club subscription which is a standard practice in theme companies. Backupbuddy is the most popular plugin from their offering and it costs 150 USD a year for developer license. The plugin club just cost 47$ more and it is a no brainer to get it.