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?