I want to display a field as read only in a `ModelAdmin` form, so I added it to the `readonly_fields` attribute. 

However, since the field contains a currency, stored as an integer, I want to apply some nice formatting it. I've created a custom `ModelForm` for my `ModelAdmin`, trying to apply the formatting in the overridden `__init__` method.

The problem is, I cannot find the value. The field is not present in the `self.fields` attribute.

Does anyone know where the values for the `readonly_fields` are kept, or is there a better/different approach?