Curry,Tendon & Onigiri for vegetarians
If you like a Indian curry, I recommend you a vegetable curry with rice or nan. More cheep japanese vegetarian food is a Tendon(rice-bowl with vegetable tempura/flitter) or Onigiri(Musubi). I think it's a fast food & ceep one.
If you want to have Tendon, Tenya（天や）shop is ceepest one(about \500～600-en),have a negetable bowl.
If you want to have Onigiri, you can get it at 7-11,Lowson,Famlymart and another convenience store.