The If/Then/Else statement required for zero checking is meant to determine if the denominator (or bottom) of a fraction is zero, prior to performing a calculation. In your example, the denominator is the Qty on Hand formula, so that should be used for the zero checking:

"if Formula:Qty on Hand <> 0 then (Formula:Actual Cost/Formula:Qty on Hand) else 0"

Of course, the numerator of a fraction can be any value, including zero.

If you also wanted your new formula to print zero whenever the Actual Cost is negative (as the syntax currently states), the you need to include both comparisons:

"if (Formula:Actual Cost > 0 and Formula:Qty on Hand <> 0) then (Formula:Actual Cost/Formula:Qty on Hand) else 0"