The reason for it's insane price is ofc due to the rarity.
- They can be obtained in "Battered Junkboxes" with pickpocketing low level mobs. Theres many spots out there such as the populare stockades due to no competition. But stockades has a lockout after 10 runs.
- The place i've been recieving tips about provides you with 250+ boxes/hr easily.
The spot is down the catatombs in duskwood, in Raven Hill.