The creatively-named Why Not restaurant offers a lunch buffet for Rs 95, or Rs 90 for take-away. This includes rice and various curries, which are fairly tasty but also on the spicy side. The seating is at plastic tables on an outside patio. They also offer accommodation.
This guesthouse has a very nice set vegetarian dinner for Rs 300. It comes with rice, three or four curries according to the menu though we were served five, plus salad and dessert. We weren't very hungry so we asked if we could share one dinner between the two of us, and they brought us more food than we could finish but still only charged us Rs 300 (plus 10% service charge).
The first juice stall on the way towards the Samadhi Buddha has very good juice. They call it orange juice, but it's made from green, unripe oranges and tastes more like lemonade. Be sure to ask for sugar, and ice if you're not too afraid of the local water. A tall glass of this stuff costs Rs 20 and is very refreshing on a hot day. Juice stalls can be found around other sites in Anuradhapura as well, though quality varies.
The receipt says Family Baker Main Street Anuradhapura although it does say Alankumala somewhere on the building. There's a supermarket next door.
Lunch was good, less than Rs 200 for all of us with sodas. It took us a couple minutes to figure out the routine. You pay first. Since we were simply ordering the vegetarian curry everywhere, we did there too.