The software is free and published under the GNU Affero General Public License v3.0.
The costs of running your own server and maintaining your domain name will vary with the providers. However, for less than \(\$100\)/year you should be able to have an adequate web server. The tutorial on this website will guide you through setting up everything on AWS for \(\$78\)/year. The cost is similar if you want to go through DigitalOcean, although you will have to purchase the domain somewhere else.
The software is written in C++. No dependency on Python, PHP, and MySQL. The installation and integration of the necessary software is free and explained in tutorials on this website. The software can easily be extended with your own C++ code.
The code autograder is executed in isolated Docker containers and the server is protected from the broken or malicious code.