A and B are incorrect: use amounts (singular) for direct costs (plural)
C : less is used for countable nouns. lower than should be used for non countable nouns
Well, after posting the answer I went through my notes and reference material.
I found the reasoning that I used in the quoted part to be wrong.
less than is used for non countable nouns and lower than for countable nouns
In this sentence 1% of xx billion $ = money. Money is uncountable ->
fewer than some money - worng (fewer is used for countable nouns)
lower than some money - wrong
less than some money - correct
The confusion is created as x $ is countable. But it is countable only if it refers to $ bills (actual paper notes). Looking at the sentence (and 99% of the time in GMAT) x $ is referring to money and not actual $ bills. hence it becomes uncountable.