Are there any extra files for the game on this site?
You mean like mods? I doubt you'd find any now seeing that the game was only released roughly a month ago. However, I am working on a modpack for AOE3, if you are interested :D
is it gonna cost me anything?
LOL! No. There are public sites that people upload their files to that you can download for free. AOEIIIFiles currently has a few files for AOEIII including a new editor but as the game gets older more files will come.