Index: product.py
===================================================================

--- ./modules/purchase/product.py
+++ ./modules/purchase/product.py
@@ -125,12 +125,14 @@
                 supplier_id = Transaction().context['supplier']
                 for product_supplier in product.product_suppliers:
                     if product_supplier.party.id == supplier_id:
-                        for price in product_supplier.prices:
-                            if Uom.compute_qty(product.purchase_uom,
-                                    price.quantity, uom) <= quantity:
-                                res[product.id] = price.unit_price
-                                default_uom = product.purchase_uom
-                                default_currency = product_supplier.currency
+                        price = product.get_supplier_price(product_supplier,
+                            quantity, uom)
+                        if price is not None:
+                            res[product.id] = price
+                            default_uom = product.purchase_uom
+                            default_currency = product_supplier.currency
+                        else:
+                            price = product.cost_price
                         break
             res[product.id] = Uom.compute_price(default_uom, res[product.id],
                 uom)
@@ -141,6 +143,17 @@
                         res[product.id], currency, round=False)
         return res
 
+    def get_supplier_price(self, product_supplier, quantity, to_uom):
+        pool = Pool()
+        Uom = pool.get('product.uom')
+
+        res = None
+        for price in product_supplier.prices:
+            if Uom.compute_qty(self.purchase_uom,
+                    price.quantity, to_uom) <= quantity:
+                res = price.unit_price
+        return res
+
 
 class ProductSupplier(ModelSQL, ModelView):
     'Product Supplier'