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I'm not sure that's correct. There is a set NHS price for prescriptions - it isn't magically the case that all drugs cost the same in the market. You'll see this if you go in with a private prescription - it will be considerably more expensive for most things.
If you are over 60 or otherwise entitled then they waive the fee - but the costs of NHS prescribed drugs are otherwise subsidised if the cost would have been above the determined prescription price.
NHS don't subsidise prescriptions unless you're over 60, have a disability, on benefits etc.