Mind you, that there are a lot of things that you can do to reduced the price, but I will mention it later.
Now, for the question that about IS IT SAFE TO TRAVEL ALONE TO JAPAN?
I would say, yes.
I've travel to Japan alone in 2013 and I have no problem.
But of course that will be back to you, because even though the crime rate in Japan is low, that doesn't mean you should let down your guard.
Don't travel alone at night, especially in quiet street.
And please avoid making trouble with the local....
Now, for Japan travel cost!