when a new pill first comes out, teh manufacturer puts a patent on it to stop other companies copying it right away. hence the premium price eg Yasmin.
After i think 3-5 years, the patent ends and other manufacturers can purchase the rights to re create that pill, therefore more brands= more competitive prices.
Pills that have been around for a long time, eg Levlen ED are covered by the PBS (Pharmaceutical Benefits Scheme). You will find that if you have a concession card from Centrelink, the pill will be available to you from a minumum of $4.60. This is because the government pays the diference to the pharmacy. so if you look on the box of your pill you can see how much the script would cost if you purchased it without a concession card.
As yet, Yasmin is not on the PBS and that is another reason for its price premium. However it is claimable through some private health insurance funds. so you can claim the cost back if your fund covers you, and the pharmacy will supply you with an official receipt if you would like to do this- you just need to ask.