I had villagers on my map in 1.6, and some trades available.
On the first screenshot you can see that I give an axe and I take back an enchanted axe WITHOUT paying emeralds (and I should pay).

Then, when trading unlocks the followind trade (created in 13w37b), you can see I have to pay emeralds to enchant this sword so it's alright.

(sorry for my english)