Wunderlist Pro is now free as of 4 April (see link below).
Wunderlist Pro gives you Ad Free, Unlimited Assigning, Unlimited Files and Unlimited Subtasks.
Available for iPhone, iPad, Mac, Android, Windows, Kindle Fire and the Web.
How can I get Wunderlist Pro?
As of April 4, 2018 we are no longer charging for Wunderlist Pro. Since we are no longer charging for Wunderlist Pro, the service is no longer available for purchase. Instead, you can simply log in with your existing Wunderlist account to access all Pro features.
Previous pricing here.
FYI - Wunderlist is owned by Microsoft. Microsoft have been building a new (and free) app called To-Do (which isn't anywhere near as good yet) but is being developed by the same team that built Wunderlist. I guess because Wunderlist will eventually become To-Do, they have stopped charging for Wunderlist Pro subscriptions. This would suggest that To-Do will likely take over within one year.