Is the only benefit to the 'cost' of a product/service to see the profit from the job or product? Our trick is we buy a lot from Amazon and the prices change often - so most of ours has no cost in it for the product. But we enter the expense as usual from credit card purchases/payments.
Ideally it'd be great if there was a true cost on every product and service - but.
I can share some info about your concern, @ryan_s1111.
QuickBooks is designed to follow the First In, First Out (FIFO) when tracking the inventory. Means, it will calculate depending on the cost of the item entered when recording it. This will not depend on the cost entered when a product is set up under the Product/Services menu.
Since there are no actual costs for the products you purchased, you can manually create the transactions for the project. Then, enter the cost or rate of the product in the specific transactions. You can also skip the Cost field under Purchasing information when adding a new product in QBO.