Ridiculous Data pricing

Without any communication to advise that contract expiring, Mtn slaps on charges for data at an unreasonable rate. Data prices are coming down, but with no advise charges on my account. When querying with customer care, the response is as follows "Please note that service fees are billed in advance therefore based on termination date, customer is billed pro-rata on discounted subscriptions and pro-rata on standard subscriptions." The average price for 1GB is R99.00, but got charged R 75.00 for 500Mb. The amount although small, its the principle that counts