Local noodle shops are probably your best bet - order a bowl of udon or soba and you'll get a pretty filling meal for not too horribly much. McDonalds and other fast food places are going to be costly - that beef's gotta come from somewhere, right?
Also, it's not very exciting, but rice is cheap and filling. Sushi and sashimi are expensive and most Japanese people don't eat it frequently. You can buy rice balls in supermarkets (onigiri, I think - gak! I'm forgetting my Japanese, help me someone!), some have fish inside, some bean, you won't know til you try it. But "sake", in addition to being the drink, is also the word for salmon. It's a pretty common filling for the rice balls. They're tasty, inexpensive and a very common dish - all the kids at my daughters' school ate them for lunch every day.
If you have a hankering for meat, you can get yakitori just about everywhere - skewers of meat in a special sauce. Yakisoba is another commonly available meal (all street festivals will have stands selling the above, and it's usually a pretty decent helping.)
For drinking, tea is probably your cheapest option - you can get bottles of (cold) green tea in a million different flavours from vending machines everywhere.
Oh, corn soup is another readily available meal, and you can also get that from vending machines - when we were there it was roughly 150 Yen for a can - about $1.25. On a cold day, it was just great - even just holding the warm can made life better. Many vending machines have both hot and cold sections (hot=red, cold=blue), so if you like your coffee hot, press the red button, otherwise, cold coffee!
:tumbleweed: