For budget eats we go to Harbourside and buy some takeaway and sit by the water and enjoy the food.
There are heaps of restaraunts on the upper level or harbourside with water views. We were quite happy with the Festival Cafe and Bar the last time we went there. Dinner meals cost between $18-28 which is a good price for the area.
Other options such as Hurricane's, Crinti's and Dragon Boat are maybe a few dollars more expensive but in the same price range with the same wonderful views.
If you look around the area most of the decent restaurants in Darling Harbour and surrounding area are similarly priced, so you might as well eat with a great view.
I have to admit that most of the time we go to Darling Harbour my friend insists on going to the buffet at Novotel, which is the best buffet we have tried in Sydney.
You cannot really go wrong with anything in Cockle Bay. My favourite is Blackbird Cafe. It might not be the cheapest option but it is definitely not unreasonable pricing when the food is good and comes in large portions! not to mention the view is stunning and all the trendy bars are nearby.