Not sure of their worth... They could be considered a rare, now that they no longer spawn.
Where they came from:
Prior to the explosion of Haven, when you created a standard paladin and did the new player quest for it, one of the steps would give you a treasure map as a reward. If you were on a normal account, it would be a level 1 map.
However, if you were on an account that was still (young), the map would be a Youthful map. Only (young) characters can dig them, and they aren't locked, IIRC. They used the same locations as the Felucca Ocllo maps, only on Haven. They would spawn "chest guardians", which were non-aggro lesser spawn (most of the same spawn as a level 1), that only attacked if you attacked them (like the undead in New Haven, currently). The chest would have loot comparable to a level 1, PLUS, if you were still young, it would have a replacement Youthful map in it.
They are still usable by new accounts, if you give a map to them, but eventually you'll get a map back from the chest that goes to one of the areas blown up, and that will be that.