No costs! All Apps are completely free. You got it right, they are free and uses no advertisements. I don't reject donations, but i don't ask for it.
You get all these free apps on Google Play - give it a try. Depending on which Android Version you are running, you can't see all apps.
Because I want to share with you my work. Android Developing and Design are what I like. If you like my work, add me on g+ and share your feedback.