First of all, a lot of things may have changed 10 years from now. Prices in Beijing will probably have gone up quite a bit, whereas prices in Tokyo might go down (in real terms). As it stands, Tokyo is an expensive place. Your expenses there would probably greatly overshadow those in Beijing, assuming the same living standard.
I guess you could get a return flight to Asia from Boston for… I don’t know, a little over $1000? Maybe half of that for a return trip Tokyo-Beijing? The question is what sort of deals you can get for one-way tickets. It seems to me that they tend to be pretty expensive – much more than half the price of a return ticket. This is the sort of thing where a travel company which makes a deal with the airlines might actually come up with a fairly attractive package price.
A lot of people do long backpacking tours through Southeast Asia but if you’re doing 4 days in Thailand I would recommend Bangkok. I have to say I think lilikoi is exaggerating, calling Singapore a country – it’s a big city, that’s all.
Overall, my advice to you would be to not wait and do everything at once many years from now. Just pick the place you most want to see and go there as soon as possible – it will be fun! :)