One of my new site (a fashion site) has been using adwords, for targetting user in Japan (30$) i notice it costs lot bigger than user in usa ( i just need to spend 10$ ). I use the same keyword for both usa & japan ads. Can anyone tell me why Japan Ads cost more expensive? Im new to adwords, so i dont know too much about it.

I'm not exactly sure but it could be a number of reasons. Maybe your Japan ads aren't optimized so you are having to pay more per click.

Another reason (probably more plausible) is that there are a lot of similar sites fighting over the same keywords. I have a japan related site and by using some SE tools I found out that I had a lot of competition. My favorite tool gave the monthly searches for a particular keyword divided by the number of sites fighting over that keyword (all japan related keywords) and the results were astounding. In short, there are a lot of sites fighting over japan related keywords.