50 dollars a day would not be doable, not if you are staying in paid accomodations. I did find one place in Osaka that was only $21 a night, but mostly I was spending around $40 a day on accomodation. If you want to do it that cheaply, perhaps couch-surfing would be a better option.
As for food and drink, I think you could do as little as $10 a day eating at soba noodle joints and lunch counters, but given the wide and incredible variety of food in Japan, I'd consider allowing yourself a higher budget than that.
When I was there, I spent an average of $100 a day excluding airfare, including food, accomodation, drink and transportation (intra-city and inter-city (JR Rail pass)).
Instead of starting a new thread, I'll add another question here. Hope someone can help.
On what budget can you travel in Japan, for example eating in cheap restaurants (possible) and staying in ryokans or so?
Is it possible for around 50 dollar a day?
If it wasn't for the high price of accomodation you'd be fine. In general Japan is expensive (I'm sure you know this) but there are plenty of places with cheap, cheap food if you don't mind noodles that is (I'm talking about ¥200 cheap). They're great though and there are plenty of grocery stores/markets if you're into making your own food or eating sugar coated goodness all day. If you're looking at staying in ryokans, the cheapest you'll get will be $40 a day plus they're a pain to find if you're in a major city (they're often expensive when compared to hotels)
You can get a lot of good Japanese food for only £3/£4. Beer is a bit more expensive but it varies a lot depending on where you are.
We visited Tokyo, Kyoto, Takayama, Hiroshima and Mount Koya (Koyasan). It depends what your interests are and whether you wanted to see a smaller side of Japan.