class Product(models.Model):
    product_name = models.CharField(max_length=250, blank=True, verbose_name='Нименование')
    def Some(self):
        return 123

class Product(models.Model):
    product_name = models.CharField(....)
    product_price = models.FloatField(....)
    product_quantity = models.FloatField(....)
   
    def my_calculated_field(self):
        return 55
   
   my_calculated_field.short_description = u'Всякая хрень'
   
class MyModelAdmin(ModelAdmin):
    list_display = ('product_name', 'my_calculated_field')