Pricing was determined by checking multiple reputable online retailers. And if you're still on the fence between the Purple and Surveillance HDD, the information on this page won't be a lot of help. The WD Purple sells for around $180, while the Surveillance HDD is slightly higher at $187. Those prices are pretty much in line with other specialty 4 TB 3.5" drives. In fact, the Purple is actually only a few dollars more expensive than the WD Red.
Once you start moving into the enterprise space, prices quickly skyrocket. While the WD Se and Re are roughly 50-80% more expensive than the Purple, the enterprise Seagate drives are on a different planet altogether. As I noted earlier, those Seagate models are in there as a performance reference; they wouldn't normally compete in the same space as the Purple and Surveillance HDD. They're 2.5" disks with much higher spindle speeds and a SAS interface, which account for the massive price premium. We just wanted to add them because the 10K and 15K Seagate offerings perform so well, considering they're rotating drives.