You can download Fixed Anvil Repair Cost on CurseForge and Modrinth:
  CurseForge:   https://curseforge.com/minecraft/mc-mods/fixed-anvil-repair-cost
  Modrinth:      https://modrinth.com/mod/fixed-anvil-repair-cost
To keep a better overview of all mods, the issue tracker is located in a separate repository.
  For issues, ideas, suggestions or anything else, please follow this link:
    -> Issue Tracker
Because of the way mod loader files are bundled into one jar, some extra information is needed to do a PR.
  A wiki page entry about it is available here:
    -> Pull Request Information
Requires the library mod Collective.
   This mod is part of The Vanilla Experience modpack and Serilum's Extra Bundle mod.
Fixed Anvil Repair Cost is a minimalistic configurable mod which makes some changed to the anvil. You can set how many levels a repair should cost, forever. The cost does not increase. It's also possible to set the amount of materials needed for a repair and how much percent an item should be repaired by an action.
Configurable: ( how do I configure? )
repairCostLevelAmount (default = 3, min 0, max 100): The amount of level it costs to repair an item on an anvil. A value of 0 keeps vanilla behaviour.
repairCostMaterialAmount (default = 1, min 0, max 64): The amount of material it costs to repair an item on an anvil. A value of 0 keeps vanilla behaviour.
percentRepairedPerAction (default = 0.3333, min 0, max 1.0): How much the item is repaired per action. By default 33.33%, so 3 of 'repairCostMaterialAmount' fully repairs any item.
Repairing an elytra with the default settings:
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You may freely use this mod in any modpack, as long as the download remains hosted within the CurseForge or Modrinth ecosystem.
Serilum.com contains an overview and more information on all mods available.
Comments are disabled as I'm unable to keep track of all the separate pages on each mod.
For issues, ideas, suggestions or anything else there is the Github repo. Thanks!