Nope, the $2 is just if you also want it to cross post your RSS out to Twitter, Medium et al. If you only want it to go into Micro.Blog, that's free. The pricing page makes it a bit clearer:

http://help.micro.blog/2018/pricing/

Basics: free

* Your own Micro.blog username.

* Replies to other posts, stored on Micro.blog.

* Add an external blog such as WordPress for posting.

* Use the iOS and Mac apps to browse the timeline or post to supported external blogs.

* No Micro.blog-hosted blog.

* No page at username.micro.blog.