pay to read a single topic
 

is there a mod that let users pay to read one certain thread?

Other than putting the single thread in it's own forum, I don't think such a mod exists.

It could be trivial to do it though. Depending on how sophisticated you need the interface.

If you set up a subscription that put the paid members in a new usergroup, you could have a plugin for showthread that checks the threadid and the usergroup and kicks the user to an error screen if they haven't paid.

Heck, you could even use template conditionals to blank out the content if they haven't paid.

It becomes more complicated if you want checkboxes and such or the ability for others to set up paid threads.

you also have to take care of printthread, archives, and showpost