The unopened ones of these sell for around 400k to 3m usually depending on which monster. Some of the rarer ones would fetch 5-15mish possibly, with things like Ancient Wyrms even fetching a bit higher. Their worth is variant depending on which collectors have which boxes. I have about 15 unopened boxes of these (unsure if I have yours.) I would guess 1-3m honestly, sorry I know it's lower than you were thinking maybe. :**( If you have a commission vendor, you can start high at 5-10m ish and lower it until it sells (commission vendors do not have a cost per day.) The terathan ones were a little more common of the monsters that spawned with them, and they had a higher drop rate than lesser monsters but were easier to kill than Ancient Wyrms and such.
Opened ones are usually the ones saught after for most collections, as they only take up 1 spot in the house. I also collect unopened ones though, but they take up anywhere from 5-40 items space each. I love these things and that's why I collect the unopened ones, but were I to need it, I still would likely only spend 3m or so at the top. I have the opened ones already of all the terathan creatures.
The unopened ones like yours are worth more than the opened ones for sure. At least 3-4x the value because of rarity. Most players opened them. : P