Coke Zero and Diet Coke are both low-calorie sodas that have artificial sweeteners. But they are different product lines aimed at different market segments and the artificial sweeteners they use are also different. Coke Zero is also called Coca-Cola Zero and Diet Coke is branded Coke Light or Coca-Cola Light in some countries.
Prices for soda vary depending upon the retailer. Here are the current prices on Amazon.com: