There are multiple valuations that can be attributed to inventory depending on the task at hand, however for a MRO, the most common valuation is the fair market value. This valuation is defined as how much it would cost to buy a comparable equipment or conversely how much the company would make if selling the equipment on a marketplace like ILS.

If you do not have access to the historical data reflecting the transactions that have been made to acquire the parts, then, the most common approach consists of going back to the suppliers asking for a quote (RFQ) for the equipment. This will give you a baseline for a new equipment. With this baseline, you can derive the value of the units you have considering how much flight hours and flight cycles they have in them (considering rotables). Similarly, ILS and other comparable marketplaces are useful to survey prices of parts.

If you have a price per unit (historical transactions), then, you can apply this per-unit price as the baseline for the valuation of the inventory. You only need to discount the flight cycles and flight hours if the unit is a rotable with codified lifespan. If the inventory is old and is intended for aging fleets, then, this price might have to be depreciated because there might never be any further demand for the inventory. Although the opposite happens once in a while: the OEM has stopped producing the part, and the price of the last units on the market skyrocket.

The MTBF doesn't tell you how much your inventory is worth. It only tells you about your expected ongoing rate of component changes.

Likewise, the lead time doesn't you the inventory value either. It only tells you how proactive you need to be to reorder "soon enough" to maintain the desired quality of service.

In general, market prices are the best indicators but getting those prices is time-consuming. So the quality of the fair market value estimate is a tradeoff between what is at stake, and the labor costs involved to refine the figures.