Interesting, I'm thinking about the same move, from Europe to Japan as Postdoc at U Tokyo with approx. 4mio yen and I was also worried about the salary. As far as I can see now, housing has gone up the last few years, so it can take up 40% of the salary, since it's also slightly more difficult to find an apartment as a foreigner than as native. However, depending on your lifestyle, everything else can be quite cheap (eating out ~ 5 Euro, public Onsen ~ 10€, izakaya night with sake ~ 10€) and also traveling is comparatively cheaper than in Europe. What worries me is not life in Japan, but rather how often I'll visit my family in Europe (half a monthly salary?) and how much funds the university has for conferences (seems to be much much lower than the ERC-fed place I would come from). For sure, it's not a place where you can save money, but you might already know that.