Thanks for posting to the Community, @sjangity.
QuickBooks should show X item QOH as 300 since you wanted to build 300 products using this item.
When you purchase this item as a bulk order, I'd suggest adding a unit of measurement so that it would appear on the PO as one bulk purchase and will reflect as 300 items in QOH items. It's like purchasing a dozen of eggs and recorded as 1 item in PO and saving it in QOH as 12 pieces.
Let me show you how when you record it in PO:
You can then Receive Inventory with Bill and Pay Bills to make the number of items appear on the QOH column.
That way, your inventory assembly items will be recorded seamlessly and accurately recorded as 300 in total.
Just in case you wanted to adjust the Sales Price of the item due to the customer's demand, you can follow these steps:
Please let me know if you have more questions. I'll keep my notifications open. Have a great weekend!