Some drugs are only available through prescription, I take a fair whack of nerve pain killers which I obviously only pay my fixed prescription charge for (I actually have a pre paid prescription certificate so it costs a bit less), but the drugs in question cost a whole lot more per pack than I am paying.
Drugs companies have huge R&D costs in the drugs and a limited period of time to recoup these cost and make a profit, hence the reasoning some drugs cost silly silly amounts. Other drugs are a lot cheaper, but they're generally the older and more frequently used drugs that any pharmaceutical company can produce due to the patent rights having expired.