Firstly, you can fly from Bangkok to Tokyo very easily. Fairly cheap, or expensive. Up to you :)
Yes, accommodation is fairly pricey in Japan. Without an exact idea of your budget, advice in that area is difficult. Options range widely, but single rooms can be found in Tokyo for around 3,000 yen up.
A week in Tokyo is great. The subway network is superb, and there are several areas of Tokyo that are all interesting in their own way.
The temples of Kyoto are really impressive, but I wasn't much enamoured with the modern city itself. I wouldn't spend 5 days there personally. I would instead spend some time in Nara, which is smaller but more interesting and atmospheric, and is home to Tōdai-ji temple, the largest wooden structure in the world. I can also totally recommend Osaka, which is kind of a smaller but funkier version of Tokyo. You could easily spend 3-4 days poking around Osaka. Osaka aquarium is one of the largest in the world and well-worth a visit.
There are of course many other interesting places dotted around the south of Japan. If you like tech, make sure you take a Shinkanssen ride (bullet train). Also make sure you check out the National Museum of Emerging Science and Innovation in Tokyo. It's in Daibo, and the ride out there is quite picturesque. And also hit Akihabara, Tokyo's main tech district if you want cheap electronics or are interested in Manga.
If you want relaxing, make sure you try an Onsen while you're there ;)
Last edited by Colonel Mustard; 25th June 2014 at 09:14 AM.