How can I set a default value for `FloatField` in django.

Initially when I had created the model and declared the float field as:

```
cost = models.FloatField(null=True, blank=True)

```

and db also created successfully through south migration.

But now when I am trying to do edit on that field using the html form, and no value is entered in the cost field, in the form its throwing `value error`.
Thus I think if I can set a default value for cost field it may fix the problem.

But I don't know how to set a default value for `FloatField` in `django`.