You need a few more columns:
- First one should contain the weight value for each item
- Second one should contain the formula: current price * weight
- Third column should contain the formula: new price * weight

Then, below your range, divide the sum of third column by the sum of second
column, then remove one. Format the result as percentage.

Recommendation: test it seriously with a few items (2 or 3) before relying
on the final result, so many business decisions having been based on excel
sheets with errors in formulas...

Stephane.