I'm planning something similar and am looking at about £3000 (~$4500 US) for two weeks. Double the people makes that 6000/9000. A hotel for 2 comes to £1500-£2000 (unless you go swanky up market and not a cheap economy room) and then flights might be up to £1500 each; that comes to £5000/$7500 total, maximum. Being flexible with the dates (look for prices for the entire week both there and back, you can often find prices halve on certain days) can be a great advantage, even if it means you might have to stay in the country a little longer. Whilst it's always good to have some extra cash just in case you go over budget, it looks like you have plenty. Your best bet is to start looking up prices though; use tripadvisor for the hotels and I use opodo for flights, but I don't know how good that is outside of the UK. In fact, I just looked up flights for a trip from London to Tokyo, 1st - 16th December and you can get a (decent) ticket for under £700 a pop.