This a classic example of how to do bad statistics complete with nice charts. A fair and a much better metric would be dollar per hour per TB. Each of these companies manufacture drives in different classes of MTBF. For example WD has drive with 1 million hours of MTBF and they also have drive with 300K hours of MTBF (which is 30% cheaper). Higher MTBF translates to higher cost. In addition you want to make sure you had considered all the discounts. For example, it would be easy to buy WD drives in higher MTBF class lot more cheaper on wholesale prices or deals than Seagate drives without deals.