yes, there is no free working ads extension available
Yes, you could write your own extension and create the functionality yourself... then provide it for free for the community. Additionally provide support in case of questions and of course keep the extension up to date in case of updated libraries or new Android APIs...
More information about how to create an extension see the App Inventor Extensions document, however that will be more advanced and will require some Java skills...
And some thoughts from @Italo
You can start learning all you need to learn to do it, then do it, after long days of trying things, testing and failing, all in your spare time, leaving other things you like to do for this.
Then after all that time and intellect invested, just give it out for free so everyone can make money with it except you.
Sounds awesome right?
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.