Price drop not communicated to customers

I have taken out a contract with Vox last year September. I have been paying R1049 per month. When I was searching on Google today, I noticed that the contact I am on, is only R899 per month. I phoned Vox and they admitted that the prices went down in July. When I asked them why the reduced price was not communicated to clients, I was told that it is the clients responsibility to check the price on their online portfolio. When I asked the salesman if they whould have notified the clients if there has been a price increase, he said yes. Why not a drop in price then as well? The sales clerk then said that he would change my contract amount, but only from this month. I feel that I should get a credit from July.