Estimating the cost of an assett that needs to be replenished I am working with hard drives.  For simplicity,  let's say all drives have the same capacity, 1TB and all cost $100.

They typically last about 4 years,  when they need to be replaced.    They drop in price about 50% over that period of time.  They cost 100 dollars for the first for years,  $50 for the next 4 years, etc.

What's the usual way of estimating the cost of something like this?
The retail price should include the total net present value of use of the item. At any given point in time, the value of the item is the purchase cost minus accumulated depreciation. Depreciation should include both physical wear and any non-physical types of depreciation such as obsolescence.