For this: "So I need that quoted price on the invoice"
You use Sales Order or Estimate. You enter the purchase and job track it as Not Billable, and here is the Cost entry. You Invoice from the Estimate or the Sales Order, and that firm price you documented for that customer is what fills in on the invoice for you.
And you still have comparison data for reporting. You just are not using the Directly Billable status from the purchase; you are Invoicing by fulfilling the sale.
There are at least 5 ways to "make the sale."