class Item(models.Model):
    name = models.CharField(max_length = 200, )
    price = models.FloatField(max_length = 10, )

class Action(models.Model):
    item = models.ForeignKey(Item)
    count = models.FloatField(max_length = 10, )
    price = models.FloatField(max_length = 10, ) #take from current value of item.price