formula to calculate savings between two numbers

Hi Guys,

This might be a silly question but I need to calculate the savings/difference between two numbers.

This would seem to be easy at a first look as e.g old number(C3) = 5, new number(D4) = 3. I could just use D4-C3=2 which is the saving. HOWEVER in my circumstance sometimes the
new number is 0, as the new product will not cost anything. In this situation a standard minus formula would not work.

Could anyone help me with this?

Column 1 = New cost
Column 2= Old Cost
Column 3 = This should contain the saving for the user.

Great! I have just realised that the new number could also be higher than the new number, which means there wouldn't be a saving. In this case is their a way to indicate that this is not a saving? I guess I might have to add some kind of conditional formatting, but not sure? e.g old value =5 new value =10... Means there is a -5 saving.