I'm wondering if anywhere in Pathfinder is a magic item that duplicates an effect similar to the old 3.5 Mordenkainen's Magnificent Mansion (or even a much much smaller version of this spell), basically creating an extradimensional and persistent living area accessible when necessary.
If not, is there any way to calculate the cost of such an item?
Mage's magnificent mansion still exists, and as such you could have a staff with it. There's also the option of a wand of tiny hut. Those are just off the top of my head as the simplest ways to do it without getting into wondrous items or actually, you know, looking it up myself.
You want an Instant Fortress. Page 520.
Sadly the Fortress is not extradimensional. The only extradimensional solution that comes to mind is the Rod of Security.
Or a custom item of rope trick.
I believe most are like Genie lamps. But then there is Kakashon, which is a major plot element in that AP
This might help. It's older but cooler than the regular options.
Lotta cool options. Most of these options aren't persistent though, correct? As in, I can't set up a nice laboratory and have it still exist upon reopening the space.
That's an interesting idea, Zagnabbit. You wouldn't happen to have the stats on such a beast, would ye?
permanent demi plane?
it sounds like you want to be able to kind of carry it around with you though right?
Something to that effect, yes.
A custom magical item with Mage's magnificent mansion is possible but expensive
Alas, so I see. Is there possibly a cheaper alternative?
Of course, you probably aren't mythic, might not be an archmage, and may not want to spend a path ability on it.
None of the three, unfortunately, although frankly that seems like an awesome option. Not playing mythic, though.
Lotta cool options. Most of these options aren't persistent though, correct? As in, I can't set up a nice laboratory and have it still exist upon reopening the space.
That's an interesting idea, Zagnabbit. You wouldn't happen to have the stats on such a beast, would ye?
No stats no.
It's basically a minor artifact that is a permanent, mobile gateway to a small, personal Demi-Plane.Pathfinder #24 "TheFinal Wish", secondary article "Decanter of Black Death"
The above article does stat that the creator made about 300 of them though, so they aren't exactly a unique or campaign breaking item.
Any permanent, inhabitable, extradimensional space that is large enough to contain several people is going to be prohibitively expensive. The ability to use it as a treasure cache is just too much in game terms (since you never need to worry about weight or storage again, plus if it's mobile you can always have every piece of mundane equipment you could ever need). A "familiar pocket" X100 maybe?
I have a home-brewed item that does just that plus some other things, it's a Staff and it's insanely expensive even with my half-assed pricing making it barely affordable for a big level player and it only provides a space that is about 20,000 cubic feet. (like a 3 bedroom house as an example).
Talk to your DM about this, if he doesn't mind a mobile spell lab it's not an unlikely get item. While it's gold cost would be staggering it can be built in such a way as to be non-game changing, beyond saving the party from nighttime random encounter tables.
In PF module "The Harrowing" there is a minor artifact called Harrow Deck which 1/day casts Gate to a specific demiplane. If you are a player just tell your GM that you want to play this.
Its not official, rather 3PP, but I created a map of an extra-dimensional object called the Amulet of Cerene, that was published as a map/magic item for Assassin's Amulet by Roleplayingtips.com - its a little pricey at $19.99 for a 300 page PDF, though.
Here's the map of the item - basically it is worn by divine assassins as a hideout and a means to hunt their prey. Its an extra dimensional space that exists within the magic item itself.
Instant Fortress is the closest item I have found to the requirements.
Creating a custom item is the best path to meet all of the requirements.