minor fix

operation.py

@@ -224,13 +224,13 @@ class Operation(Workflow, ModelSQL, ModelView):
 
     def get_unit_price_w_tax(self, name=None):
         Tax = Pool().get('account.tax')
-        res = 0
-        if self.unit_price and self.accommodation:
+        res = self.unit_price
+        if self.unit_price and self.accommodation and not self.taxes_exception:
             values = Tax.compute(
                 self.accommodation.template.customer_taxes_used,
                 self.unit_price or _ZERO, 1
             )
-            if values:
+            if len(values) > 0:
                 value = values[0]
                 res = value['base'] + value['amount']
         return round(res, Operation.total_amount.digits[1])