models.py

class OrdemServico(models.Model):
    funcionario = models.ForeignKey(Funcionario, verbose_name=u' Nome do Funcionario', on_delete=models.PROTECT)
    cliente = models.ForeignKey(Cliente, verbose_name=u' Nome do Cliente', on_delete=models.PROTECT)
    servicos = models.ManyToManyField(Servico, verbose_name=u'Servico ', on_delete=Models.PROTECT)
    data_entrada = models.DateTimeField(verbose_name=u' Data de Entrada ', default=datetime.now, blank=True)
    data_saida = models.DateTimeField(verbose_name=u'Data de Saida',  blank=False, null=False)
    valor_total = models.DecimalField(max_digits=7, decimal_places=2, blank=True)
    
    def save(self, *args, **kwargs):
        total = self.servicos.all().aggregate(total=models.Sum('preco'))['total']
        self.valor_total = total
        return super().save(*args, **kwargs)