Will, 
My daughter just finished studying a semester in  Edinburgh & she recommends the following: 
'Mother India Cafe' & 'The Mosque Kitchen', both of which offer good Indian food at a very reasonable price. 
'Ting Thai Caravan',  has reasonably priced Thai food which I enjoyed very much when I visited her. 
Holyrood 9A, has very good hamburgers & fries. 
Lunch preferences: 
'Oink' for delicious pulled pork sandwiches. We ate here too:-)
'The Pie Maker', for a variety of savory pies-- very inexpensive.  
'The Black Medicine Coffee', is good for sandwiches, paninis & bagel sandwiches, as well as a variety of drinks, including coffee, smoothies, etc.
(I enjoyed a delicious lemon cake here too!)
'Union of Genious', a good place for soups, cakes & snacks.
Enjoy your trip, Edinburgh is a beautiful city!