Yes, WoW Token is cheap than WoW Gold, but WoW Token has 10 limitations.
1. You only can purchase a maximum of 10 Tokens per week with real money from the in-game shop. But you are free to choose gold amount from a third party.
2. It may take several days to sell a Token on the auction house for gold. But if you want to get wow gold anytime, you can buy it from a third party.
3. Tokens purchased from the Auction House can only be consumed for game time or Blizzard Balance. But you can use wow gold to buy game time, pots, flasks, enchantments, mats, repair, expensive mounts, pet items, and etc.
For more details, you can click here: https://www.mmogah.com/news/wow/why-many-players-prefer-to-buy-wow-gold-rather-than-buy-wow-token