The government really dropped the ball by letting carriers decide how to finance cellular number portability. They should have regulated the charge and made it a per-use charge. So if I want to change carriers, I should pay something like $25-$50 out of my pocket to cover the expenses. Now we have carriers charging all customers whatever they feel like for this. It's the "Universal Service" charge all over again.
Gizmodo : The results are in: what the carriers are charging each month for portability