Cost is a property of individual enchantments, and is used by vanilla in combination with environmental aspects like tool enchantability and bookshelf count to determine the final cost of an enchantment as shown in an enchanting table.
Having access to the base cost of an enchantment using these vanilla methods can allow plugin developers to determine the "value" of an enchantment, and use it in custom logic where needed. I came across this recently when trying to assign an economic value to enchantments during tool repairing, and noticed these values don't seem to be obtainable under the current API.
Upstream has released updates that appear to apply and compile correctly.
This update has not been tested by PaperMC and as with ANY update, please do your own testing
Bukkit Changes:
fb23cbb3 Define surefire plugin version
d022084a Define ordering for MetadataStoreTest
99a7f6f0 PR-910: Match generic max absorption attribute name style with the rest
c7390d71 PR-909: Update tests to JUnit 5
CraftBukkit Changes:
f0661c351 PR-1230: Move unstructured PDC NBT serialisation to SNBT
452fcb599 PR-1256: Update tests to JUnit 5