I think part of the difficulty is that you are thinking that a mathematical expression or equation has only one concrete meaning. Numbers are abstract, and can be applied in different ways in different situations. Also, an expression is just an expression; the meaning of 1/2 * 100 is simply "the number that, multiplied by 2, gives 100", namely 50.
One way to think of 1/2 * $100.00 is to think of dividing $100 into two equal parts, and taking one of them. The expression doesn't literally mean $50/$100, but the fact that 1/2 * $100 = $50 is related to the fact that $50/$100 = 1/2. This is a ratio that says 50 is related to 100 as 1 is related to 2. None of this says that 100 actually consists of two parts. One application of the ratio would be that, say, one item costs half as much as another, so you could say that for every $100 you spend on the latter, you spend $50 on the former. There are no "parts" involved.
The fact that 2 * $100 = $200 merely means that $200 is twice as much as $100. The ratio $200/$100 = 2, in one application, would mean that for every $100 you spend on the second item, you spend $200 on the first. Again, no "parts".
Applications of math determine what math should be used. The math doesn't imply anything about the real world.