Why is that a flight with Qantas from Dubai to Sydney costs more than 5 thousand Australian dollars,
while if I fly from London, it takes only as low as 5 hundred English pounds.
What is the cause for this price difference?
There are exactly two airlines that fly Dubai-Sydney direct, namely Qantas and Emirates. They operate closely together with code sharing, cross booking etc, and have zero incentive to cut into each other's margins. Any alternatives will involve a stop somewhere along the way, with all the extra time and hassle that involves. This is why Qantas can charge a premium and get away with it.
By contrast, you have literally a dozen choices for London-Sydney: not just Qantas and Emirates, but British Airways, Etihad, Turkish, Singapore, Thai, Malaysian, Air India etc. The Qantas option via Dubai is not clearly superior since it also involves one stop, and arguably others offer better service, so it has to compete on price. And this is why Qantas is comparatively cheap.
This is a typical situation in pricing of airline tickets. The airlines compete on a concrete destination (e.g. London - Sydney or Dubai - Sydney in your case). They charge more for the Dubai - Sydney ticket because they can - i.e. there is not much competition on this particular route and they presumably offer a direct connection. On the other hand there are lots of options if you are flying from London to Sydney (you can basically go via a number of Asian cities like Hong Kong or Singapore). From this perspective it doesn't matter that the longer route includes the shorter one, because you as a passenger can't really take advantage of this fact.
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