Your profile indicates you are already in Japan. Since your asking this question I'm going to assume you don't have anything but a tourist visa. If you had a student or working holiday visa I doubt you'd be asking about only a month.
If your already in Japan on a tourist visa then you cannot legally work. If you do work and are caught you can be deported and bared from future entry into Japan. Even if you are not bared it is probable that you will have to go through many more hoops to gain entry the next time you come to Japan. For instance having to apply for and receive a visa via the Japanese Embassy in your country instead of just showing up at the boarder with a passport. Also remember that even if you are not caught, work for a month and return home, that you can still run into trouble. If you come back to Japan at a later time and they discover you worked during your last visit on a Tourist Visa then they have reason to deny you entry again. How do they know you aren't going to work again this time? It's possible you get away with it and are never caught. For all I know it might even be very likely that you are never caught. But the consequences for if you ARE caught can be a royal pain. And not just for traveling to Japan either. A rejection in Japan might cause your home country authorities to become interested in you and begin checking out your life deeply. Or if may be that later on you want to visit, say Australia or the US, and they find out (which they would) that you were denied entry into Japan. They are then going to ask questions and may deny you entry based on the fact that you may try to work illegally. After all, you did it in Japan.
I see you have been asking about pickup tips. Honestly, if your down to 50,000 yen you can probably forget the adult fun. Your trying to pickup locals instead of P4P but your still going to have to pay for love hotels, drinks, food ect. That will blow through 50k amazingly quickly.
If you just want to stay in Japan for a month and don't mind hostels or capsule hotels, eating very cheaply ect you might stretch that 50,000 for a month. But forget about women. I mean it. Too expensive. Also, I hope you already have a return ticket to home. If not I really recommend you take your 50,000 and buy a ticket home. Save up more money and come back in future. Honestly, I don't even think I'd try to live a week in Japan as a tourist on 50,000. But I'm not the student type willing to backpack and stay at hostels.