The cost of food at an airport differs from airport to airport.
In Los Angeles, I managed to purchase a small soup (of decent size) plus a small bag of chips for about USD$5.
At Nadi, Fiji airport $10 got me 2 pieces of toast, 1 large bag of Twisties (cheese chip like snacks) and a small bottle of juice for USD$10. There only seemed to be 2 purveyors of food at the airport, one in the middle and a small cafe near the back left.
Fiji bottled water is the cheapest drink you can buy and there didn’t seem to be any water fountains.
This is what you can buy for USD$10 at Fiji airport at 6am:



