I drink mineral water (big bottle that I fill my smaller bottle with). The only 'sweet' drink I have even when not travelling is coffee, so that's easy for me. I try to avoid soft drinks because I'm at the age where I'm starting to complain about how sweet they are.
Breakfast at McD: 200 yen for a coffee and burger. It's enough to fill me up. Some people like Onigiri in the morning but I can't do rice then. It may seem lame, but I really love the tinge of wasabi in their mayonnaise. I would do coffee at my friend's place to save even more, but she doesn't have a coffee machine anyway.
Lunch at any convenience store/supermarket: mainly Onigiri because I quite like this for lunch. Or I pop into one of the rice bowl stores and get a bowl for 300 yen or less (or 400+ yen for a full set, which is very very very filling).
I can do this because I'm cheap; I survived for 3 weeks on peanut butter + jam sandwich for lunch when I was travelling in Europe (cheap + not a fan of cuisine). I don't suffer, if that's what people think. I must like what I eat, and I have a tendency to be able to eat the same thing daily for a period of time.
That said, I did vary my diet a bit in Tokyo though, mainly for dinners though because I met my friend daily at restaurants and pubs for food so that adds up. And all the crepes and Calbee cafe pig-out sessions on the weekend at Harajuku. Thus the need to budget for breakfast and lunches!
Anyway back to topic, the cheapest I have found is Scoot for S$350. Since I'm flying from SIN to Taipei, so I'm going to call them up to see how much it would cost to change my departure date and tack on Tokyo. Hopefully I'll get an interesting quote!