Purchase Order Price / Drop Shipments / Gross Profit & GL Impact

50% of our business consists of drop shipments from our suppliers.  We're having trouble calculating the profit margin on the Sales Order / Invoices of drop shipped items. 

Challenge 1:
Most of the items we purchase vary in cost from PO to PO depending on factors such as volume, contracts, promotions, deal registration, etc. 

NetSuite doesn't seem to have a method to manage multiple cost structures on the purchasing side for an item (like a drop down price level on the Sales Order side). 


We've got around this by setting up discount items, but it is far from ideal.  The main issue is that the discount items can't be accounted for in the margin calculations on the Sales Order/Invoice.  The options for cost estimate are Purchase Order Rate, Purchase Price, Item Defined Cost, Last Purchase Price.  Selecting any of these options will flow through the wrong cost on the Sales Order and Invoice.  And we pay commissions on profit....so it is imperative that our invoices display the correct profitability.  Any ideas on how to achieve?