How much money will I need to spend while WWOOFing?

It is hard to generalize and will depend on numerous things such as the kind of personal effects you buy and the frequency with which you buy them, how often you make long distance telephone calls and the length of those calls, how much you move around from host to host and have to pay bus and train fares, and more.  While WWOOFing with a host, all your meals and accommodation costs are taken care of by the host.   The cost of living in Japan is on par with the US, Europe, Australia, New Zealand, Canada, etc.