I think that will be coded into the server files rather than stored in the database, it's something like the max + any wep or gear can have or damage cap, level cap etc which is coded into the files. Not come across it in the database anywhere. But it would be nice to be able to edit it to some value higher such as +6 for example then i think more people would buy them.
Seen as most servers have higher alchemy rates now. Before astrals used to be more valuable than immortals but now it's the other way round with higher alchemy rates.