I've authored commercial membership plugin memberwing a while ago. Total revenues including consulting fees and subsequent sale of it as a business (with more of subsequent consulting) are in 6 figures. It's a good strategy of free version for promotion of commercial version although Wordress is strict in terms of how much you allowed to pimp your commercial stuff through their free plugins repository.

If your plugin helps people to make money - price tag could be higher vs. utility plugins.

To protect my commercial plugin from disassembling I've developed my own PHP encoding system (zencrypt) that worked in 100% of all hosting accounts.

Paypal is a popular payment processor to sell through although nothing prevents buyer from buying and then requesting refund next minute and Paypal will not always be on your side for digital products. I've created my own commercial license tracking system wrapped in encoded PHP that also warns about expiring license and asked for renewal. If buyer pulls "refund" trick on me - I've deactivated license remotely and that disabled plugin for offending customer.

Consulting was a big source of revenues as some customers wanted to add this and that feature and some ended up paying $10,000-20,000 per lifetime for extras. This is pretty impressive considering that PHP is not exactly big money making skill.