One of the reasons many people believe foreign folks come to Japan is to make big bucks. The general impression among Japanese folks is that we make a lot more money than they do. That's a half truth. If you're an underpaid Japanese office lady with a crappy job or a university student toiling at a convenience store or McDonald's for minimum wage, then that assertion is correct. If you do nearly any other full-time job which requires a university degree (and all us teaching types have to have degrees because we can't get a work visa without one), then it's probably not correct.
Most Japanese people calculate your wages based on what they pay to be taught by you, not on what you are paid. They assume you get a much bigger slice of the pie than you do. My referral agency charges students 4,900 yen an hour, but I get paid 2,800 yen an hour. I'm not complaining about the pay, mind you, but you can see how a student might think that I'm getting a great deal more than I am.
It isn't really how much you make but how you live which dictates how well you live here. One of my students is a nurse who makes less money than the average starting level foreign teacher salary of 250,000 yen. However, she also lives in a dorm provided by the hospital which only costs 22,000 yen a month and gets free meals at the hospital cafeteria. To put that in perspective, a super cheap accommodation in Tokyo for a teacher would be 50,000 yen a month for a shared place (gaijin house or shared apartment). If you want your own small place, you're likely paying 70,000 yen a month or more. In terms of food expenses saved, most people can't eat for much less than 30,000 yen a month unless they live on ramen. That means that the "perks" of my student's job as a nurse add to her salary to the tune of at least 70,000 yen each month.
My underpaid student who is a nurse told me that she spends 50,000 yen a month on clothes and she spends that much every month. She gives her old clothes away to friends and coworkers, but she has a constant revolving door on her wardrobe closet. She also goes to concerts and bars with friends at least once a week. Despite having a low income, she's certainly not hurting for discretionary funds.
After you live here for awhile, you understand that Japanese society is different from Western society in that jobs tend to take the average life circumstances of a person into account when deciding how much to pay. Most women who work as nurses are single and child-less because married women with children can't put in the sorts of hours required or be on call. Most office ladies are either married and working for spare cash or are still living with their parents so they aren't paying rent or shouldering the burden of supporting a family. Men who marry get raises compared to single men and men whose wives give birth to new people also often get raises.
Since life circumstances dictate how Japanese folks tend to be paid, but not how foreigners in usual foreigner jobs (teaching, translating, entertaining) get paid, it's hard to compare salaries. We are paid what the market will bear based on supply and demand. They are paid what their culture feels is appropriate based on their life circumstances in many cases (though certainly not all). Companies assume that you won't apply for a job which doesn't suit your needs and everyone "knows" that certain jobs are at a low pay level for students or housewives looking to support discretionary spending and no one expects adults who need to support a family to do them.
This is all my roundabout and tangent-heavy way of saying that how you live is more important than what you make in many cases when it comes to money. In Japan, the wages tend to reflect how they expect someone doing that job to be living. That's not to say there aren't people who simply do not have enough money to live comfortably, even in Japan where unemployment stats are low and it is currently a market (on the Japanese side, not the foreigner side at present) which favors job seekers over job providers.