Cheap means budget supermarkets and food courts; expensive means organic groceries and restaurant dining.
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At home sets your weekly grocery basket: cheap ≈ A$70, moderate ≈ A$110, expensive ≈ A$160 (for all 21 meals — reduced by meals eaten out, and by ~60% for homestay). Eating out sets the per-meal price: e.g. dinner cheap ≈ A$18, moderate ≈ A$32, expensive ≈ A$55.