Here in Perth I have had no luck getting Dextrose at any supermarket or stores like Big W. However I go to a Home Brew store and can buy it at below $4.00 a kilo or, if I want to buy in bulk 15kg for just over $30.00.
At my local Health food store they tried to sell me Dextrose at $31.00 per kilo!
Its definitely worth checking prices.