Repair costs are not balanced with vanilla repair costs. For example.

• You can repair a broken sword fully from broken to max health (significantly more expensive)
  OR
• Fix the item slightly, (using one item) and then repair the nearly broken item to max durability (Much cheaper)

ruined diamond sword full repair: Costs 6 diamonds & 15 levels
ruined diamond sword Alternate repair: Costs 5 diamonds & 6 levels (using 1 to slightly fix it, and 4 to fully fix it)

Thanks for the report. I'll get around to fixing this soon, but I wouldn't consider this insanely high-priority since Mojang's repair code has major discrepancies in repair costs as well for vanilla items.

Fixed in revision 86fa192 - version 1.1.1 has been published to Releases & on Curseforge.