All add-ons must be distributed free of charge. Developers may not create “premium” versions of add-ons with additional for-pay features, charge money to download an add-on, charge for services related to the add-on, or otherwise require some form of monetary compensation to download or access an add-on.
Paying someone to design and implement an addon which then is free of charge for anyone else might be acceptable. The “no charge for services related to the add-on” part is debatable. I assume that’s more about services related to an existing addon, not initial development of it.
In practice, if you pay for the development of the addon, and then it becomes free of charge for anyone else, that’s kinda like an initial, up front donation, which is fine.
I’m fairly sure Blizzard will not care about this particular case, as long as the end result adheres to their policies. But to be safe, you can email and ask them at WoWUI@blizzard.com.