We already posted about all the changes coming to Hearthstone [Free] this year as the Year of the Mammoth card rotation hits with the release of the next card set, and one tiny detail in the avalanche of information that caused a few of us to raise our eyebrows was the full-cost dust refunds Blizzard is doing for gold Classic cards that are getting shifted to the new "Hall of Fame." The idea is cards that instead of nerfing cards that are a little too powerful, or are restricting design space, they're just shifting them over to Wild and giving their full dust value back to players. We reached out to Blizzard to clarify exactly how this all works, and it turns out if you've got the dust right now to craft a bunch of gold cards, you can effectively get them for "free."
Once it officially becomes the Year of the Mammoth, Blizzard is going to scan everyone's card collections and refund dust based on what you have at that time up to the playable maximum in a deck- Regardless of whether you've had them for years or you crafted them the day before. These are the cards that are getting refunded:

Azure Drake
Sylvanas Windrunner
Ragnaros the Firelord
Power Overwhelming
Ice Lance
Conceal

Blizzard will refund based on the maximum value of the cards you have, so it will prioritize giving you dust back for gold cards over the normal cards. Say you have 2x gold Azure Drakes and 2x normal Azure Drakes, you'll be getting 800 dust back, or 400 for each of the gold card...