IF([Amount]==0.0,0,ROUND(([Sales Price]/[Amount]*100),2))

Using this formula, it checks to make sure if the denominator is zero, if so, it will return a 0.
If the denominator is not zero, it will calculate the decimal, take it times 100 to return a percentage and then display the result out two places.

If the percentage is desired in the displayed column, add the following column and hide the one above. This formula assumes the name of the formula above is P %

CONCATENATE([P %]," %")

Choose the type of text for this formula and it will take the result of the first calculated column and add a space and a percentage symbol.

 

Example with both formulas combined:

CONCATENATE(IF([Amount]==0.0,0,ROUND(([Gross Profit]/[Amount]*100),2))," %")