Most new users start their websites with X-Cart 5, not X-Cart 4.
So, with X-Cart 4 the only way to earn money is maintaining existing stores. I'm not sure how you get the -825%, but this may be the case for X-Cart 4 modules (depends on the module popularity, of course).
To profit from developing X-Cart modules, you are to create them for X-Cart 5, not X-Cart 4.
To help learning X-Cart 5, we extend and maintain a knowledge base (
https://devs.x-cart.com/) and answer questions in the forums. Also, for existing X-Cart 4 module developers we can provide some personal lessons (please contact us).