I ran through breaking and placing a lectern several hundred times, and Mending, Frost Walker I and II, and Curse books always costed an even number of emeralds. There should not be an odd-even bias, as it doesn't make logical sense and no other enchantments have this.
Steps to reproduce:
- Spawn villager
- Place lectern
- Check enchanted book trade
- Break lectern
- Repeat several hundred times
The cost of a treasure enchantment is never an odd number of emeralds.
(It is very unlikely, but possible I am wrong about this statistically speaking, as is the any case for anything regarding random chance)