When someone clicks the ‘+ NEW TOPIC’ button on my site (once registered and logged in), it should open up a payment gateway that has to be completed before that person is then directed to the Discourse post topic page.
The two payment options so one-offer:
a) A one-off charge of £25 to post one article.
b) Rolling monthly subscription charge of £75 to post unlimited articles.
A workaround is to use an existing payment plugin and have people added to a group that has write access to the category in question. The monthly option is easy, as removing a user from the group when the subscription is over is a standard feature. Removing the user after a single topic is created is a bit more complicated, but possible in a plugin.
You’d likely need a budget of $500-1000 for a custom plugin.