It is 100% free... IF you are fine with the code being put open-source (anyone can see the code) and I believe in your idea. Otherwise, I charge an hourly rate of $70. This can be lowered if I believe in your project or not.
I am a devoted Python and C developer, cryptographer and pentester who has created every app you see here. Every app is written in Django or Python, although I can use most common web languages, like PHP or Node.js. I have been using Ravencoin and blockchain for over a year. You can see my projects in GitHub and read my blog.