One of the selling advantages of the PureDisk appliances is that you only buy the de-dupe licenses once.  All hardware eventually gets old.  When that happens, you can buy another PureDisk appliance and re-use your existing de-dupe licenses.  You could build your own appliance with off-the-shelf hardware and re-use the licenses.  With a DD, when you upgrade the hardware, you're re-buying the software again since the prices are not separate.

When you're doing the cost comparisons, factor in not only the initial purchase but also subsequent purchases.

I despise per-TB licenses too (as somebody else pointed out),  If I had a choice, I wouldn't buy them and would tend to avoid the PureDisk licensing model specifically for this reason.

   .../Ed