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?
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
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."