title: admin django. Add the total price in admin
body: <p>hello I have to add a line or for example total_price = 123312 in admin</p>
  
  <p>models.py </p>
  
  <pre><code>class Exit(models.Model):
    description= models.CharField(max_length=50)
    data_uscita = models.DateField('data uscita')
    price = models.DecimalField(decimal_places=2, null=True,blank=True)
  </code></pre>
  
  <p>admin.py</p>
  
  <pre><code>  class ExitAdmin(admin.ModelAdmin):
       list_display =['description','price','total_exit']
  
  def total_exit(self, request): 
      total = Exit.objects.all().aggregate(tot=Sum('price'))['tot']
      return total
  </code></pre>
  
  <p>but is not ok because j have a columns with the total_exit that are repeated. I want total write only just once</p>
  
  <p>I'm using Python 2.7.11</p>