Hehe... inexpensive lol. I have a bridge I can sell you (that's a Sydney joke by the way).
For brekkie... go to a supermarket and buy cereal and milk or bread and jam... seriously, otherwise you'll be out $10 for a coffee and slice of toast.
Most Bars and Pubs/Hotels have lunchtime specials... if you're lucky you can get $10 steaks, chicken schnitzels etc. The drinks will set you back a bit more on top. The Helm, or Shellbourne, or Cargo Bar have decent, if not spectacular food near darling harbour. Not a big fan of the food at a couple of the Irish pubs though...
Dinner you'll be hard pressed to stay under $20 per person if you eat out at most restaurants. Macquarie Park is other side of the city to me... haven't been up that end of town before