This isn't a simple transaction. Most users with larger avatars, special usertitles etc are given them by admins as a special favour. There is no set price or specific set of actions to follow. You'll find most admins will turn you away unless you are willing to pay ridiculous amounts of TC because, let's face it you don't need a larger avatar.
Speaking as an ex-admin I would turn away nearly every user who asked me and frankly found it quite annoying when I was bombarded with these requests. I would especially make a note of turning away users without VIP as they are cheating the system so to speak.