Hope people don't mind me going back a few pages, because I wanted to look this up and get it right.
6. What is roughly the highest value artifact can get? Few hours ago I received topaz scepter covered with cow bone/random gem/aluminum worth 140k. During 6~ forts I've made which lasted 6 years or longer this is most valuable one I've seen, usually they're 10k. Sad dwarfpanda.
6. I honestly have no idea what the upper limit of value is. I think there is an upper limit of value based on the base material used, but if you for example have an artifact adamantine statue encrusted with gold and diamonds, the value can get absurdly high.
Then again, maybe a cap was set in place by Toady for balance reasons in the more recent versions, I don't know for sure.
In my current fortress (in the current game version), I have been granted Oslanoshot, "Windvanished," a raw adamantine mechanisms (hooray for grammar). Ingredients include the initial raw adamantine, and then melanite, hungry head leather, rope reed fiber, adamantine (not raw), yellow zircon, raw adamantine, raw adamantine, raw adamantine (that last one is a depiction of Tobul, the deity of metals), adamantine (not raw), and copper (hooray for an anti-climax). Value: 2,568,000☼.
I also have four artifacts that are "adamantine," not "raw adamantine," including two that are worth over a million. The 2.5 million still tops them.
So, "is there a cap"? I am willing to say "no." Items in the millions are beyond any practical benefit already, and it would serve no purpose to have a cap somewhere above that.
--Rexfelum
Edited P.S.:
Over here is an artifact worth over 3 million as the result of a glitch.