I guess not many. In my experience, website owners will most often opt for free solutions, if they meet their requirements, or if their website is large enough, they will get a custom solution from companies that provide enterprise level solutions. Usually this way they get a solution tailored to their specific needs, and also get other services along the way.
I wouldn't agree with you, I would say most serious website owners pay for what they need to make their site make money. why not?
For demo or assaying ok to use free versions or limited, but it is a very low number of such useful plugins is free.
to OP, I will suggest you consider the possibility of an option where you will charge a percentage of the realized sale. it's better than a demo, reduces the initial investment for your potential customers, and if they have good traffic and sale, they can always buy the full version.