Pro Rating

My bill is due on the 3rd of each month but my data starts over on the 6th . I want to decrease my data from 30GB to 15GB but without a large pro rate. Can you let me know when is the best time? I've changed on the 6th, the 12th, the 25th and with no success. My recent changes was on the 12th and I had to pay $187.00. Is there a way to avoid the large amount or is it just a way for At&t to squeeze money out of its customers.

Help Me please!!!!!!!