Symphony.

I'm building a website that should have a constant (a price) that can be edited by authors. This constant should be auto-filled in when creating a new subscription in the backend (and it is used on some places in the frontend). How would I implement this? Using a custom field added to the preferences pane + a custom field?

You could use the Static Section extension to create a section named Settings or Configuration, and just allow a single entry to be created. It won't solve the auto-fill in the backend though...

If it's specific to subscriptions (and you have other rules too, like expiry?) then you're best writing your own field. You could extend the normal text input field to add this extra functionality.

You could try to add the field's value to the parameter pool, then use the Default Event Values extension to set/overwrite the value when the entry is saved from the frontend.

Great, thanks for your suggestions. I think I can make it work like that.