Similar to how Apple and Microsoft require you to buy a developer 'subscription' for $99. The google play costs $25, does Ubuntu have a similar cost or is it free?
The important part of the answer can be found here:
For commercial software you have to register, but:
"The Ubuntu Application Developer programme is free of charge, there are no fees to join."
Of course if you want to sell your Apps, there will be a percentage of charge:
"The application publisher chooses a global price to sell their application for. For each sale Canonical retains a transaction fee of 20% after tax. If you sell an application for 10.00 USD, then the fee will be 2.00 USD."
You can use the Ubuntu one but it goes through a review process https://wiki.ubuntu.com/AppReviewBoard/Submissions
Or you can use Launchpad and you run your own repository that other users must add to the Ubuntu software center