In addition to the MW alkahest box fare, all strongboxes opened in-game during this period will have a chance to drop either 200 master enigmatic scrolls or the Dark Mark head slot cosmetic. Spooky scary skeleton head, anyone?
Do keep in mind that each masterwork alkahest box contains 16 alkahests each, with the possibility of getting up to 16 more each time you open one.
With that in mind:
Considering the standard price for MW alkahests on the trade broker (40g to 60g), each box will have to sell for about 700g to 800g at the vendor to make the value of the boxes and the alkahests themselves sort of equivalent.
500g per box would be a steal, while 1000g would be reasonable due to the chance to double-up. Let’s hope En Masse doesn’t plop them up there for more than 1000g and call it a day.
I myself will be logging into TERA to grab some boxes, if only to secure myself some easy enchantments when I come back later. Like I always do.