Django set field values dynamically based on other fields

I'm trying to set a field default value based on other field selection that is foreign. Basically, these are the classes:

class Product(models.Model):
        description = models.CharField('Description', max_length=200)
        price = models.FloatField('Price')

class Sell(models.Model):
        product = models.ForeignKey(Product)
        price = models.FloatField('Price')

Each "Product" has a default price (or suggested price), so when the user, in Admin pages, want to add a new Sell and he/she choose a product, I need to copy dynamically from Product.price to Sell.price the suggested price. I cannot use "save" method because the user can change at that moment.

Is it necessary to use JavaScript explicitly? or Is there an elegant way in Django to do that?