Well you need to understand that the carrier an iPhone is locked to has normally subsidized its cost originally. If you have fulfilled a (usually) two year contract with them, they might be willing to unlock the iPhone without any additional cost. However, if that is not the case they might want to recover some of that subsidy if the user wants to unlock the iPhone without having a contract with them or before the required amount of time.
You don't say if you have been looking at your carrier's charges or at any other unlocking site. Please note that your carrier is the owner of the lock and any attempt to unlock the iPhone, other than by this carrier, is an unauthorized modification of the iPhone and could create further problems for your iPhone.