Est. Extended Cost in the Sales Order/ Invoice is Zero when Cost Estimate Type is Purchase Order Rate

As per field level help, Purchase Order Rate - Initially uses the Preferred Vendor Rate cost, and then after a purchase order is entered, this type uses the most recent actual purchase order rate. Special orders and drop-shipped items use this cost information."

This means that if the sales order is not drop shipped or special order, the Purchase Order Rate would remain zero for that sales order. Thus the Est. Unit Cost and the Est. Extended Cost would also be zero. Purchase Order Rate only uses the rate of the linked drop ship or special order Purchase Order.