You will need to SUM the amount, then this will work.
So your calculation would be:If sum([Amount]) < (-100) then "over 100" else "under 100" END
I'm gonna guess the issue here is that you're coloring by individual values when you want to aggregate it, then assign a color. Since every individual gas expenditure is <$100, it remains green.
ALSO, since you're dealing with expenditures, your aggregation should be looking for values beneath -$100.
I was able to arrive at a simple LOD calculation which did the trick for me:
Drop this in as your color and assign the colors you prefer.
Please give this a try and see if it works for you!