As mentioned in a comment, you have a problem with that of.
- In my previous organisation we were not doing so much work.
- In my previous organisation we would not do so much work.
Sentence 1 strongly implies "actually, we worked very little" or "we didn't work as much as this", while 2 means "if we were asked to do this, we wouldn't have done as much work on it as you/we/they are doing now" or possibly, "in comparative situations, we did not do as much work on it as you/we/they are doing now."
Honestly, on a gut check level, as a native speaker, my first roguish thought was Sentence 1 is the one you use to insult your previous job. ("We were worthless lazy slackers.") Sentence 2 is the one you use to insult your present job. ("You guys are working too hard. There's a much easier way to do it.")
Let CNY = the cost of living in New York, and CT = the cost of living in Tokyo.
Sentences Nos. 1 & 2:
The cost of living is twice as high in New York as it is in Tokyo.
The cost of living in New York is twice as high in Tokyo.
these mean: CNY = 2 × CT
e.g., if it costs $500 per week to live in Tokyo, it would cost $1000 per week to live in NY
Sentence No. 3:
The cost of living in New York is twice higher than in Tokyo.
I would say this one is a non-standard way of making the comparison, and, of your options, it would be better to avoid this one.
Sentence No. 4:
The cost of living in New York is two times higher than in Tokyo.
Again, that's CNY = 2 × CT, but remove the than:
The cost of living in New York is two times higher in Tokyo.
and it means 2 × CNY = CT
e.g., if it costs $500 per week to live in Tokyo, it would cost $250 per week to live in NY
Sentences Nos. 1a & 1b:
Your child is a hundred times as adorable as your pet dog.
Your child is a hundred times adorable than your pet dog.
The first one is okay, but the second one needs an extra word:
Your child is a hundred times more adorable than your pet dog.
Sentences Nos. 2a & 2b:
Her room is twice as large as mine.
Her room is twice larger than mine.
Once again, I'd avoid the second one. However, when dealing with cost, we can use the expression twice as much as:
The cost of living in New York is twice as much as in Tokyo. (CNY = 2 × CT)
Best Answer
The first is grammatically incorrect. At the very least it should be "additional cost which is to be incurred" or "additional cost which was incurred". In that case the emphasis is on the fact that the additional costs have or will actually occur. In the second sentence the emphasis is on explaining the reasons for the additional costs - whether those reasons actually arise and the additional costs are actually incurred is not addressed. Personally, the most terse and elegant way of stating this in my opinion is just:
"Your following payment shall cover also the additional cost incurred due to the reasons stated in my previous letter"