Here are some tips for getting cheap food in Aguas Calientes
You can buy food by eating where the locals eat. There are some food stands by the soccer field and yesterday I got a cheeseburger with fries for 13 soles. I also got some grilled beef on a stick for 5 soles for dinner. It was so delicious.
There is also like a cafeteria above the produce market (Mercado Aguas Calientes) You have to go to the third floor. Make sure you pick a place that has lots of people sitting there and the food is being made fresh and hot in front of you.
My friend got rice, schnitzel, soup, and it came with fries and salad for 13 soles. This was for lunch. They said the food was really delicious. They didn't eat the fries cause rhey were cold, and they didn't eat the salad (worried about uncooked food).
So you don't have to buy the expensive touristy foods. Sometimes those tourist restaurants will mark up the price if they see the hotel you come out of.
It's also cheaper to buy the big bottles of water, like the 2.5 liter ones and use it to refill your water bottles than buying the 1 to 2 liter one.