It should be $/month of the third party app developer or a tiered plan where you choose depending on the number of users using it.
An easy win-win-win-win for Twitter, partners, users and developers.
So if I create a free Twitter client that has 10000 users, I will have to pay Elon 80000 Dollars every month? Seems a worthwhile effort ;-)
I don't think Twitter cares about free Twitter clients, or even commercial ones.
It's always amazed me that TweetDeck has existed for so long, but now if users aren't being monetized by ads or paying for (Blue) features in the Twitter app I can see them not being welcome on Musks Twitter.
TweetDeck survived as it's owned by Twitter
Sorry meant TweetBot!
Your users can pay. This is the "insert API key to continue" model, standard across the industry.