Higher density doesn't always mean lower prices
Write wear and speed aren't a major issue.
Firstly: large disks have far lower percentages of writes, which means the sparing load is much reduced.
Secondly: Samsung stepped back to ~40nm architecture when they went to 3d, specifically because it's a lot faster and more robust (and cheaper to fabricate)
Spinning rust drives _are_ less reliable than flash ones, the failure rate at work is testament to that (thousands of drives, flash fails at a rate of less than 1% of rust)
Now that Sammy are offering 3-10year warranties and almost all the spinners come with 12 months, the math favours flash even though rust is much cheaper upfront.