I think this is the cheapest at the moment.
Features:
Online Play : Enjoy competitive and co-operative online gaming with friends and rivals from all around the world in supported Nintendo Switch titles, such as Mario Kart 8 Deluxe and Splatoon 2.
Nintendo Entertainment System – Nintendo Switch Online: Enjoy access to a growing library of classic NES games anytime, anywhere – with added online features and other enhancements, like HD resolution, filters and special save states!
Save Data Cloud: Securely and automatically back up your game data online for easy access. This makes it simple to retrieve your game data if you lose your Nintendo Switch console, or start using a new one.
Smartphone app: Use the dedicated Nintendo Switch Online smartphone app to enable voice chat and access special features in supported Nintendo Switch titles.
Exclusive offers for members: Enjoy a range of offers only for members, such as exclusive products that only Nintendo Switch Online members can purchase!
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Chief Bargain Hunter