If you're referring to the threads on this forum, $75 is for many of the UK carriers.
In most places that aren't America, there is more than one carrier offering to sell iPHones. This being the case, each carrier is willing to do a factory unlock on the phone for a fee. In certain countries, this is $75 or so.
In Australia, Virgin Mobile does it for free because they're pretty awesome in my opinion.
If you are in America, and someone is offering to unlock your phone for a fee, you're being conned as there are ONLY software unlocks (as distinct from factory) which are always free.