I think $3600 for Japan trip is not enough for 20 days.
What kind of lodging are you thinking off? Hotel in Tokyo about yen 7000.
Hokkaido hotel about yen 6000. Osaka yen 6000 t0 7000. These hotels are nearer to mrt or train station. Further away hotel is slightly cheaper but then you will be wasting time moving and tired by heavy photographic gear. If you plan to go Disneyland and Universal studio then better factor entrance fees into your budget.
14 day JR pass only for JR trains and cost about S$750. Other six days on your own. If your 14th day end in Osaka and you need to get back to Tokyo by shinkanasen, ticket about 12,000 yen.
Food also not cheap. Cheaper udon about 400 yen. Bus ride also not cheap. With a crowd may be you want to visit shinjuku at night. Allow for some beer allowance.
Otaru is also nice for sea food, esp the large crap. But I rccomend the fried chicken shop near to the Otaru station. Not exp and very tasty.
Winter is Hokkaido is beautiful. But go in the middle of Dec so you can see more snow. And hopefully can catch some snow fall.
Hope you manage to work out your trip within your budget.
$3600 is certainly enough... in fact, i'm quite careful with planning the budget...
i can find lodgings in hostels located in Asakusa central Tokyo for 1500yen per night (Khaosan Tokyo Smile) and lodging in Kyoto (5 mins from Kyoto Station) for 2000yen per night (Tour Club) all these are my hostels of choice for cleanliness, accessibility and friendliness after having stayed in most of the budget hostels in Tokyo and Kyoto
there are numerous food chains scattered around which offer full sized curry rice for 350yen each and other affordably priced food
14days JR pass costs around $600 and we'll be planning to depart Kyoto for Tokyo on the last day of pass validity
Disneylands arent a must in the itinery as i would not expect adults to be too into such disney stuffs... we'll be mostly visiting shrines and temples which either carries no entrance fees or a 500-600yen entrance fee
buses are certainly not cheap for travelling within the city but sure offers a great sightseeing opportunity... buses on the other hand are cheaper compared to trains for inter-city travels e.g Shinjuku to Fuji Go-Ko
as this is not designed to be a purely photographic trip, i would suggest all participants to travel light with minimal amount of photographic gear to maximize mobility... i travelled with my D80 with an 18-200mm VR lens and a tripod if i'm planning to do long exposure or night shots, if not i leave my tripod in the hostel most of the time...
i hope this year would see a greater amount of snowfall, 2006 was absolutely horrible =(