Copied from a comment I made on a similar post before.
Green Chompy Mage was sold as an Amazon Exclusive in a bundle of five Senseis. This was released some time after the other four in the bundle were already released and after the Christmas variant had already been released. The bundle didn't stay up on the site for long before selling out. Most figured it was a timed retailer exclusive - I can't really think of a non-variant that released only as a retailer exclusive?
So, to have a green Chompy Mage, it would have required:
Able to purchase from Amazon.
Willing to spend $70 on Skylanders at once.
Either not having been a big enough fan to have purchased the other four in the bundle already or being a big enough fan that you did not mind having to buy duplicates of the other four.
Wanting the basic version of a figure that you already had (or could very easily obtain) a variant of.
Wanting all five of the figures in the bundle (or willing to purchase all five when you only wanted one to four of them).
Willing to essentially pay full price (or 20% off if you had Amazon Prime) when the figures were regularly B2G1 at other retailers.
Not being willing to wait out the perceived timed exclusivity of the figure or believing it was 100% exclusive to this Amazon bundle.
Falling into all of those above categories and making the decision to pull the trigger while the set was avaliable in the short window it was (I want to say days. I don't even think it was a week.) on the site.
Then, after someone fell under all that criteria, for the item to end up through a reseller (especially a third party reseller like CeX), you'd have to have someone who met all of the above criteria and now wants to get rid of it (and in the case of a third party reseller like CeX, willing to sell it for half of what it sells for on a site like eBay).
Meanwhile, while it was a very limited supply and short window. the pack with Robow was sold on store shelves. Any kid with a Christmas giftcard could have walked in and been like "gimme that!", played with it for a week, then never touched it again until their parents were cleaning out the attic and found this bin of old video game figures that are just taking up space.
https://us.amazon.com/Skylanders-Imaginators-Villains-not-machine-specific/dp/B01LVTHWM5
That's why one is significantly more expensive than the other.