I think the best way to do that would be to offer a free plugin with added benefits once you pay for it (like buffer). I look at is similarly to buying an app on a phone, it would largely depend on how useful it would be, how much I would use it, and if I could justify paying money for it.

I have a plug in to remove the ads on the Pandora page, and to show a pop up with song name and artist regardless of what webpage I'm focusing on. I wouldn't pay for that plugin, but a strong ad blocking one could be worth the money.

Do you have something in mind?