Not true. Sometimes expensive? Yes, ALWAYS? ... NO! Many times you can get stopovers in SIN for just the price of the extra airport taxes/charges which transit passengers are exempt from. As an example, I've done return from Athens to Melbourne with stopovers in Singapore in both directions ( the first stopover was a brief 2 weeks, the second a longer 10 month stopover

), and only cost about $60 more than through fare.
To answer
stripakis's question there are several reasons why a stopover may be expensive:
- Fare conditions do not allow it, therefore what you are building is a series of one way flights. However, from Europe, many fares do allow stopovers
- The length of overall stay away is too long as the fare has a maximum duration (and the stopover puts you over the limit)
- The fare class or fare bucket that the overall fare (that permits free stopovers) is not available on some of the flights. Given the dates of travel, late Jan/early Feb, this is most likely explanation, as there is a lot of Australia bound traffic from SIN (students returning) that means flights tend to be very full and this means cheaper economy seats, or economy seats full stop, are not available on many flights. For example between 30 Jan & 5 Feb I could not see a single economy seat available for travel between SIN & MEL. Stopping in Singapore means you are looking at availablity for just SIN-MEL, going through all the way without stopping sometimes means there is different availability (for example seats that they are only willing to sell in cheaper fares for people connecting from long haul flights).
The only solutions to this problem I could see are:
1) book as far as Perth, as it has most availability from Singapore (or if lucky Brisbane, it has a handful of seats on different days) , in one direction, take a domestic flight and return from Melbourne in the other direction, or
2) book as far as Singapore and purchase a separate ticket on different carrier to get to Melbourne.