For 1 drive, the differences are perhaps not much. When you have a cluster with 2k HDD drives, the difference in handling both squential and random workloads (see my other comment) can add up.

A 2.5x difference in IO/s adds up quickly.

As for HDD prices, neither 12TB nor 8TB were top of the line at the timeframes you mention. If you buy largest available drives, or a model lower, the prices have been going down. Not as much as we all would like, but overall the trend has been moving in the downward direction.