diff --git a/uncloud_vm/models.py b/uncloud_vm/models.py
index e8504a6..3eb46b7 100644
--- a/uncloud_vm/models.py
+++ b/uncloud_vm/models.py
@@ -97,11 +97,6 @@ class VMProduct(Product):
                 RecurringPeriod.PER_30D, RecurringPeriod.PER_HOUR],
             RecurringPeriod.choices))
 
-    def __str__(self):
-        return "VM {} ({} Cores/{} GB RAM) running on {} in cluster {}".format(
-            self.uuid, self.cores, self.ram_in_gb,
-            self.vmhost, self.vmcluster)
-
 
 class VMWithOSProduct(VMProduct):
     primary_disk = models.ForeignKey('VMDiskProduct', on_delete=models.CASCADE, null=True)
@@ -115,9 +110,6 @@ class VMDiskImageProduct(UncloudModel):
 
     """
 
-    uuid = models.UUIDField(
-        primary_key=True, default=uuid.uuid4, editable=False
-    )
     owner = models.ForeignKey(
         get_user_model(), on_delete=models.CASCADE, editable=False
     )
@@ -166,7 +158,9 @@ class VMDiskProduct(Product):
     """
 
     vm = models.ForeignKey(VMProduct, on_delete=models.CASCADE)
-    image = models.ForeignKey(VMDiskImageProduct, on_delete=models.CASCADE)
+
+    image = models.ForeignKey(VMDiskImageProduct, on_delete=models.CASCADE,
+                              blank=True, null=True)
 
     size_in_gb = models.FloatField(blank=True)
 
@@ -181,22 +175,16 @@ class VMDiskProduct(Product):
 
     @property
     def recurring_price(self):
-        return (self.size_in_gb / 10) * 3.5
-
-    # Ensures that a VMDiskProduct can only be created from a VMDiskImageProduct
-    # that is in status 'active'
-    # This might not be need in "for billing only" constellations
-
-    # def clean(self):
-    #     if self.image.status != 'active':
-    #         raise ValidationError({
-    #             'image': 'VM Disk must be created from an active disk image.'
-    #         })
-
-    def save(self, *args, **kwargs):
-        self.full_clean()
-        super().save(*args, **kwargs)
+        if self.disk_type == VMDiskType.CEPH_SSD:
+            price_per_gb = 3.5/10
+        elif self.disk_type == VMDiskType.CEPH_HDD:
+            price_per_gb = 1.5/100
+        elif self.disk_type == VMDiskType.LOCAL_SSD:
+            price_per_gb = 3.5/10
+        elif self.disk_type == VMDiskType.CEPH_HDD:
+            price_per_gb = 1.5/100
 
+        return self.size_in_gb * price_per_gb
 
 
 class VMNetworkCard(models.Model):