I am monetizing a wordpress plugin. It did not started with a plan. My advice is that if you have an idea that can be implemented outside wordpress too, try to build that instead, not because you can reach a broader market, but because you can really market it.
It is true that wordpress users expect stuff to be free. I built a plugin, free at first because I had an idea. I left the plugin to float around and noticed that it got some traction. There was an opportunity. I made the plugin better, nice interface, etc, and started to get some users. I was getting 100-300 new downloads every day. Thinking how to enhance the plugin I had an idea that needed to request something from my server. Since connecting to my server means that there are other costs involved, it seemed natural to ask for a fee for this advanced feature.
First I made a "PRO" version, I had a few sales but created a lot of confusion because there were always 2 versions. Then I implemented an API key system to the free one. The user enter the API key and he get that feature. The plugin is still at 100-300 downloads per day, I don't do any other kind of promotion. The only traffic that goes to my site where they need to go to buy the subscription.
Right now I haven't started to promote it hard because I thought that making a wordpress independent version of that feature is better and right now I finished building it.
I think good plugin developers can make money from wordpress plugins if they offer some extra features paid.