Does the Sword of the Planes have a basis in the rules for its GP cost for its abilities?

Question

As per the SRD:

This longsword has an enhancement bonus of +1 on the Material Plane, but on any Elemental Plane its enhancement bonus increases to +2. (The +2 enhancement bonus also applies on the Material Plane when the weapon is used against elementals.) It operates as a +3 longsword on the Astral Plane or the Ethereal Plane or when used against opponents native to either of those planes. On any other plane, or against any outsider, it functions as a +4 longsword.

Strong evocation; CL 15th; Craft Magic Arms and Armor, plane shift; Price 22,315 gp; Cost 11,157 gp and 5 sp + 893 XP.

Empathis mine. The 315 GP is the regular cost for a Masterwork Longsword, which is needed to make a magic weapon. 2000 GP is the cost of a regular +1 Longsword. But this leaves 20,000 GP unaccounted for.

Where does this cost come from? It is not a multiple of the cost of the other enchantments. Normally the cost is determined by the weapon bonus squared multiplied by 2000 GP. But for a +2, +3 and a +4 enchantment (8.000, 18.000 and 32.000 respectively) this does not add up to the 20,000 GP or a multiple thereof (58,000). Is there a basis in the rules for this that I have overlooked, or was this an arbitrary number made up during the design process?

Answer 1

This is probably not definitively answerable

The numbers that you quote are all correct, and you haven't overlooked anything. There is no overarching design principle at work for precisely how the designers came up with that number. We can make guesses, but I would be very surprised if the justification for that price was ever made public.

My best guess would be that they took the average price of the three enhancement bonuses that it could have (58,000/3 = 19333), rounded up, and called it "good enough".

A large number of magic items are like this. For example, the sustaining spoon costs 5400 gp, when the magic item formulas state that making a use-activated, once per day item of create food and water should cost 6,000 gp and feed and water 15 people. The spoon can feed 4 people and can't water them, so it should (by the formulas) cost no more than 1,600 gp. As the magic item pricing rules state:

Not all items adhere to these formulas directly. The reasons for this are several. First and foremost, these few formulas aren’t enough to truly gauge the exact differences between items. The price of a magic item may be modified based on its actual worth. The formulas only provide a starting point.