Some addons used to require payment, such as Carbonite.
Many addons request a donation on their site, but a donation is optional, for if you feel a product's development is worthwhile to you.

I believe there are also leveling guides, which are contained in an addon, which require payment prior to gaining access. But in that case, the addon isn't so much an addon but rather a leveling guide which you access from within the game. Not sure if that is in violation of Blizzard's terms or not.