This is a common theme that occassionally pops up in both the Qantas and Singapore Airlines threads from "newbies" to those airlines, shocked at the lack of ability to upgrade (cheaply) on the airlines. It is probably no co-incidence they are both (arguably) two of the more profitable airlines in the world today.

There is one airline operating in the Australian (international) market that does offer cheap upgrades - that being Emirates, however anecdotally it appears they only offer them by invitation (or at least only have them available for requesting) when the economy cabin is full/overbooked and business cabin much emptier - ie. to make a bit of extra $$$ when they would be doing op-ups anyway.