class A(models.Model):
    name = models.CharField(max_length=64, null=False)
    value = models.IntegerField()
    ...

select avg(case
        when (value > 0 and value <= 50) then 0
        when (value > 50 and value < 70) then 50
        else 100 end)
from A
where ...

extra(select={'avg_field': case_when_query})

aggregate(Avg('avg_field')),

qs = A.objects.extra(select={"avg_field":
                     "avg(case when...)"}).filter(...).values("avg_field")

qs[0]["avg_field"]

class A(models.Model):
    name = models.CharField(max_length=64, null=False)
    value = models.IntegerField()
    average_field = models.IntegerField(default = 0)

    def _get_average_field(self):
        # Trying to match the case statement's syntax.
        # You can also do 0 < self.value <= 50
        if self.value > 0 and self.value <= 50:
            return 0
        elif self.value > 50 and self.value < 70:
            return 50
        else:
            return 100

    def save(self, *args, **kwargs):
        if self.value:
            self.average_field = self._get_average_field()
        super(A, self).save(*args, **kwargs)

A.objects.filter(...).aggregate(avg = Avg('average_field'))