There are lots of fast food chain near Shinjuku south exit and east exit. Such as Yoshinoya or Matsuya (Gyudon, beef rice bowl) or Fujisoba (soba), Hanamaru Udon (udon), Hidakaya (ramen, gyoza) , Cocoichi (Japanese curry), etc. At those places, you can fulfill your stomach from 400 yen - under 1,000 yen.
For lunch, restaurant floor on LUMINE department store maybe useful. They have various kinds of restaurants, from Japanese to Italian to Korean.