Step aside y'all. I got this
Could just be different sellers. The more expensive one is the last one that guy has in stock, so he might think someone is willing to pay a premium. However, word of caution. The first link you provided might be mislabeled. When I clicked it, I got a notification at the top of the page that I purchased that on May 2, 2013 for $19.99. Mine is definitely not that combo and it was only a single disc 3D blue-ray. I could be wrong, but I never heard of this version coming out. It's completely possible that I missed it. I haven't really paid attention to any of the new releases seeing as I've bought Avatar 3 times now.
- Blue-ray/DVD combo theatrical edition (no 3D)
- Director's edition with extended theatrical re release and like 10 hours of bonus content (no 3D)
- Theatrical 3D release, single disc (the first one you linked)
Unless you're hell bent on getting the 3D version (completely understandable and worth it), I'd highly recommend this version;
https://www.amazon.com/Avatar-Extended-Collectors-Blu-ray-Worthington/dp/B0044XV3QY/ref=sr_1_1?s=movies-tv&ie=UTF8&qid=1507724154&sr=1-1&keywords=avatar+directors+editionI'd recommend buying both if you're a fan of the film. The bonus stuff is excellent. You can even watch the whole movie as it was filmed in the volume.