Update November 3, 2018: Blizzard has announced at Blizzcon 2018 that World of Warcraft Classic will be included in the standard World of Warcraft subscription. In other words, World of Warcraft Classic will cost $14.99/month to play. Blizzard offers pricing discounts to players who pay for 3 or 6 months at a time.
The official cost for World of Warcraft Classic has not yet been announced by Blizzard. Current rumors (unconfirmed): Some people guess that players will pay the standard $15/month subscription to be able to access all World of Warcraft servers, both current expansions and WoW Classic. Others suggest that WoW Classic might be free but have an additional cost for an updated client with modern graphics, similar to the model used by Blizzard for Starcraft Remastered. Some people have proposed that WoW Classic will have its own entirely separate cost/subscription, independent of the current WoW expansion. In any situation, it’s likely that you’ll be able to use your Battle.net account balance to pay for the game or subscription. Have your own theory? Comment it below!