I am pretty sure that stuff is server sided. Even if you changed an items price to 0, it would still server sided be the correct price. What you're doing with any pure hfs file is client modding. It doesn't always effect server side completely.
PS: Adding items or modifying prices or what not would apply to their policy on hacked spawned items and what not, and would thus most likely end in a quick check through the logs to get banned.
All things aside though I'm not knowledgeable enough to get an accurate enough answer from.. Maybe someone who has a lot of experience can figure it out or let you know if its possible. Good luck!