Start out with definite fixed costs.

1) Intercontinental shipping - at the absolute best figure 2k euros for Europe to North America, as much again if not more for Singapore-Alaska (assuming it's container shipping? Where will you wait for the boat to arrive? Both Singapore and Alaska are expensive places to just hang out.)

2) You will probably need a carnet du passage, that's ~3-5k euros tied up in the deposit. Sure you will get that back at the end, but for now you still need to take that chunk out of your ready cash.

3) Going through China even once (with guides) will be in the thousands too.

Some notes on your route, from one who has not been RTW himself but has done some thinking about it...

1) The Dalton Highway to the top of Alaska - from everything I have heard about it, it's a long and difficult, expensive and not necessarily enjoyable or always beautiful slog, for the bragging rights of saying you've ridden as far north as possible. Since you're in Europe anyway, why not ride to Nordkapp instead? It's as far north or more, and the road up there is either beautiful (all-Norway), or fast and easy (mostly-Finland or mostly-Sweden).

1.5) Alternatively, if you want to ride to the north of North America, consider the Inuvik-Tuktoyaktuk road, recently completed for year-round use - sounds a lot more pleasant than dodging oil-equipment trucks all day...

2) Have you researched permits to get into Bhutan? From what I know, they are not very overlander-friendly, or tourist-friendly in general.

3) You've also got two separate entrances into China, both of which will be big expenses - and I'm not sure it will be easy at all to find a tour company to run you from the Bhutan border out to the Vietnam border...

4) Getting into Thailand with your own vehicle will be very difficult, they seem to have changed the rules to be as bad as China. (But, to be fair, Thailand's rules change often.)