Your prescription charge almost certainly doesn't cover the cost of prescribing and dispensing the medicine - even if it were supplied to the pharmacy for no cost at all.
The rationale for charges, and charge exemptions, is wayward and odd, but the worst charge should be £104 a year. Surely anyone who has any idea they will need more than twelve prescriptions a year would be mad not to purchase a prepayment certificate (PPC)?
Being exempt and often reading of others being gleeful at their exemption status, they are somewhat less happy when they realise the fairly modest advantage it gives them.
The unlucky ones (financially) are those who would have been better off with a PPC but didn't know that when they started getting prescriptions. For example, someone who gets three items in one go, then nothing, then some more items, and so on.