Well, I guess that depends on yourself.
Why do you want to go to Japan?
If you want to go shopping or head to Disneyland / Disneysea, then you should head to the travel's agent.
All you got to do is pick one of the program that they offers and enjoy yourself.
That's because most program heading to Japan usually consist of Disneyland/Disneysea, Ginza, Universal Studio and the shops.
The cost is about 15,000,000 IDR - 20,000,000 IDR, more or less.
However, if you want to go travelling yourself, you can see a lot more things and can go on your own pace.
And it can be cheaper than going with the travel agent... And also a lot more fun, especially if you like shrine and museum.
Considering that when I take travel agents, I got voted out by the rest of the tourists who want to go shopping instead heading to museum / local attraction places.
So, let's start counting on how much you should spare for going to Japan...
From Jakarta to Japan, it takes about 8 - 9 hours of flying non stop.
Can be longer if there's a stop here and there.
Lots of air flight service available, from the budget Air Asia / Jetstar to ANA.
Pick your choice, depends on your budget.
IMO, Airplane is the one that cost the most from your budget.
There's a big hype in Indonesia about free visa when visiting Japan in upcoming months.
However, it is not confirm yet... at least for now.
The cost of making visa to Japan is about IDR 320,000.
Lots of people are complaining about how difficult it is to get a Japan Visa, but as long as you fulfilled the requirements, I think you'll be okay.
Most people assumed that food in Japan is expensive, but that's depends on how you eat.
I limited the budget of 1500 Yen each day for breakfast, lunch and dinner.
Lunch or dinner can take the biggest proportion of the budget, depends on yourself.
I usually skipped lunch, because I was too excited going here and there.
So I just have snack, like Pocky or Onigiri, cost about 110 yen - 200 yen.
To make things easier, you can buy IC Card for train uses.
It's more practical than buying ticket, and you can use the IC Card again when you're going back to Japan.
You can fill the IC Card easily using the machine located at each station.
The rate for the transportation is depending on how far you travel from one station to the next.
You can calculate it easily by using hyperdia.com.
For example, you are from Tokyo Station and want to go to Akihabara.
It cost about 140 Yen, it also explain that yo only need 3 minutes to reach it.
Hyperdia really helpful, since they also tells you the time when the train comes.
Now, if you are from Ginza and want to go to Makuhari Messe, where TGS usually being held...
Makuhari Messe is located on Kaihimmakuhari Station, and you need to go to Tokyo Station first.
Since Kaihimmakuhari is located quite far, the cost is quite expensive.
The cost is about 1,230 (non-reserved seat) - 1,750 Yen (reserved seat).
If you have the schedule beforehand, you can calculate the cost of the transportation beforehand and can fill the IC Card accordingly.
If you intend to stay longer than a week in Japan, you should check the Japan Railway Pass.
Where do you spend a night then?
If you want a cheap place to rest, then check out hostel.
They are pretty cheap, cost around IDR 300.000 - 500.000, of course with shared bathroom.
If you're not willing to have a shared bathroom, you can also spend a night at the hotel.
However, Hotel cost extra and it is better if you share it between friends.
The lowest price for hotel is about IDR 700.000 - 900.000.
Ryokan is expensive, but if you have spare money and want to experience the traditional Japan, then go ahead.
6. Entry ticket for museums
If you want to go to shrines or museum, sometimes you have to pay the entry ticket.
There are free shrines, you can look up to it at the internet.
Check how much is the ticket price, so you can calculate how much you need later.
Now when you have finished calculating all of the important expenses, you can calculate your pocket money.
Yes, the pocket money that you need to buy all those Japanese stuff that you can see on the internet.
Akihabara, Ikebukuro, Shinjuku, Ginza, Osaka, Kyoto and many others places have interesting stores that worth to visit.