For the Pathfinder RPG, the costs and limits of a Bag of Holding are defined pretty clearly. Though it comes in four different types, I'll take a Type I Bag of Holding as an example. The Type I bag weighs 15 pounds, has a Contents Limit of 250 pounds., a Contents Volume Limit of 30 cubic feet, a Market Price of 2,500 gp, and a Cost To Create of 1,250 gp. The creator needs the Craft Wondrous Item feat and the spell Secret Chest. Caster Level is 9. The bag of holding opens into a nondimensional space.
For my campaign I've created a Mutex Bag of Holding. A Type I Mutex Bag of Holding is actually two different bags (A and B). Both bags open on to the same nondimensional space. Only one bag may be open at a time. If Bag A is already open, Bag B refuses to open and cannot be opened.
The advantage here is that Bag A can be across the globe from Bag B; it provides a way of resupplying a party from across vast distances, passing notes or potions or rations or whatever. Picking up and holding the bag does not provide any sort of identification, location nor communication with the owner of the other bag, but there's aways Sending spells. Passing notes through the bag is permitted.
EDIT In case it is relevant, the Player Characters in my campaign currently have only one of the two bags and don't know who possesses the other bag: friend, foe, or absent-minded wizard who left Bag B open.
Does something like this already exist? I have not found it in my searches to date.
How do I figure out the Cost To Create for this item? I treat the two bags as one set. I plan to put a premium on this because the pair allows some new creative uses over a single Bag, but I'm not sure how to cost this out, as it approaches the convenience of a Teleportation Circle that is both permanent and portable.