I'm not sure if you're going to be satisfied with this answer but for the items, the same as for the houses:
It is worth as much as someone pays for that.
There are a lot of different Worlds in Tibia and the prices are different everywhere. Some may say it's worth 15k TC while others that it's at least 30k TC.
First of all that depends on what server the item currently is. I don't think if "who signed it" is any important.
Collectors spend a lot of money on such items because usually they can afford it. They might say it was worth it but player totally not interested in such items will surely say it was not.