Panharmonicon is a lovely new piece of tech from Kaladesh.
An article outlining a really nice deck featuring the new artifact as its centerpiece recently went viral and apparently had some success on camera, here's an article on the deck: https://www.mtggoldfish.com/articles/against-the-odds-five-color-panharm....
I think Wizards made Panharmonicon Rare instead of Mythic for the simple reason that they knew it would be extremely popular and would be played as a 4-of. At mythic, this spike would have taken them to $30 apiece, at which point a playset for $120 constitutes a too-large buy-in for even veteran Standard players. If the price stabilizes around $12, or even $8, the rarity was chosen correctly. It's the same kind of reasoning that made Mutavault a rare, and not a Mythic.
This seemed like a sleeper to me, but apparently it's got the goods for Standard so there will be no under-cover era for Panharmonicon.
Some stores online still have them around $3 each. That won't last beyond today. It's looking like $8 is the a semi-stable plateau at this hour, above and beyond the data below.
If you're late to the party, and can't find non-foils under $5, the Foils are lagging considerably and are still good value. They were around $10 yesterday and they're just finally drying up around $12, headed to $20 short-term and $30 long-term.