Here is my interpretation.
(Emphasis mine; I left out an important part that will be discussed below.)
If someone has already built an addon, they are entirely forbidden to charge you money for it. The rule above says that clearly in three different ways.
(This is the part I left out earier, emphasis mine again.)
Similarily, someone cannot contract with you any kind of “subscription” for continued use of an addon that is built or modified in any way for your use.
A prospective developer cannot advertise in-game (or even here on these forums, arguably) a desire for donations or payments of any kind. However, donations could be sought on addon distribution websites
I would infer that offering to pay for building an addon is also therefore okay… on one of those websites. But you are only paying for the development effort… once built, the developer MUST give you a permanent royalty-free license to use the addon to your heart’s content.
And, importantly, the developer CANNOT charge anyone else for the same development effort. (That would be maintaining a premium addon; illegal above.) If they want to distribute the finished product, they must do so freely (said three times above).
To your question about gold… I assume that if real money is allowed on Patreon then why wouldn’t gold be allowed? But honestly I don’t know. And Elvenbane’s suggestion of an in-game ticket is smart.
Source: https://us.forums.blizzard.com/en/wow/t/ui-add-on-development-policy/24534
PS. I feel like I need to add this before someone jumps to very wrong conclusions about me and then starts a forum battle:
I do not accept donations for addon development. This post is only to emphasize that certain things are definitely against TOS, and indicate what things might be permissible.