Australia must not have been "into" the boombox culture as much as America was. So there are only a few citizens like yourselves who see the value of a boombox and are willing to pay for it. The rest think they're worthless.
Here in America, prices are way higher, because there are tons of small businesses and pseudo-entrepreneurs who gather and re-sell things for a profit. Witness that on US Ebay it's getting harder and harder to find boxes with a low buy-it-now. This is because almost everyone knows they can get big money for it.