Hi Antoine, it is true that Adobe does not sell or maintain older versions of the software, so if you want to "crossgrade" between operating systems natively then you need to be up to the latest release of Creative Suite...
One other option would be to look into running PC programs on your Mac via virtualization or dual-boot software, though that would require more work/expense.
Lastly, on the pricing, don't forget to take taxes into account. The U.S. pricing isn't quoted with taxes included, whereas European prices typically are (significant VAT).
Even apples-to-apples, there is still a price difference between different countries of course, and Adobe has addressed why this is.
Hope this helps.