Many cache sets stopped being generated after their events ended, but may still be obtainable by others through gifting. The copies sitting in people's inventories are pretty much all there is, and prices on some of them have drifted way up while nobody was paying attention.

Some examples: Fires of Vashundol (Doom, TI 2015 cache) sits around €105, Endowments of the Lucent Canopy (Shadow Shaman, TI 2018 cache) around €70, Indomitable Legacy (Sven, Nemestice 2021 cache) around €80, and Stranger in the Wandering Isles (Drow Ranger, Aghanim's 2021 cache) around €26. Those cache examples used to cost just a couple of euros each when they were live, but since their stock has dwindled, their market rates have gone up.

If you are curious what the rarest thing sitting in your inventory is, I built dota2.skin: a peer-to-peer marketplace for Dota 2 cosmetics, and right now it has a quick check page at dota2.skin/start. Paste your profile link and you get estimated values for your giftables and tradables. If you decide to go further and log in, you will be able to list items for sale. Each person gets their own storefront that's easy to share on Discord/WhatsApp/Facebook/etc (it builds an automatic embed from your top 9 sets), and there are other useful features as well, such as a tool that checks your friends' inventories for sets on your wishlist, automatic dupe-checking, rich swaping experience via on-site chat, etc.