Forcing developers to pay is the most developer- and user-hostile thing they could do, and is the main reason I don’t use Safari anymore (there just isn’t any worthwhile extensions for it).
What I’d bet most people (including myself) do is search for extensions (assuming you don’t already know the exact one you want). My experience has almost always been that the extension I want isn’t available for safari (just chrome and Firefox), or is a shitty/old version of the chrome one.
So even if the $100 keeps some more junk out than on chrome, I still don’t get to use the extensions I want and the ones that are available aren’t great (albeit better than junk).
Google requires developers to pay them $5, once, before they can upload extensions to the web store.
They’re just covering the fees to prove you’re a person/developer, and basically nothing more.