Need to BEGIN tracking inventory value

Hi!

  This may be challenging!! How do we begin tracking the inventory value of products we assemble? We've been tracking inventory quantities on the components,  removing and adding as needed without inputting costs. It creates a nightmare when trying to figure out inventory value at year end.  We'd like to enter the costs of all the inventory (components) we just counted at end of year for a clean slate January 1st. 

Background: 

We assemble products (such as soap dispensers) from 2 inventory components (bottle and pump), and sell the final assembled products alone or in sets. 

We reduce the inventory components from stock as they go out via quantity adjustments. I.e. we sell a plastic bottle with a pump (an assembled soap dispenser), we remove both components from inventory.

We use sales receipts for groups of sales (i.e. each pay period from marketplaces like Amazon). We just enter the product as an inventory item called "Sales". 

How do we do start entering the costs for our components,  so we at least can add them to inventory with costs as we purchase in the future, and have the value automatically subtract when we reduce them from inventory? 

We could start scratch by archiving and recreating inventory products with values- but won't that count the COGS twice? (We already have our expenses accounted for as COGS s we purchased components, they're just not tied to inventory items.)

Hope that's clear,  and I hope someone had a solution!

Thanks!

Christine