You may run into a promotional offer or "limited offer" for subscribing to Dragon Riders Club while playing the game. Depending on your playtime and progression, you may be one of the lucky ones to get a pop-up for this offer (or, alternatively, see it in the Events > Limited Offers window).
 
With this promotion, you will have the opportunity to subscribe to Dragon Riders Club for a discounted price of $3.99 USD per month for the first three months of your subscription. Once these three months expire, the regular subscription costs of $9.99 USD will apply. Keep in mind that this is the only promotional offer present for Dragon Rider Club subscriptions. Once this promotion has been used, it will not appear again.
 
For more info on what the perks of a Dragon Riders Club subscription are, please refer to "What are the benefits of joining the Dragon Riders Club?".
 
Note: If this offer has been claimed in the past using the same linked account, it may appear in-game. However, the checkout confirmation will show the regular price. Please be mindful and verify purchases before confirming.