The expression you have used is correct. However, you see 0 % since the previous year number is matching with current year numbers (no change).
Try with different numbers then you see the change.
Small suggestion, each and every product will have different ASP, so trying to show it by year is not correct. Use product as well as dimension then it will make more accurate.
Calculate ASP by product then sum it for that particular year.
Hope this helps.