it costs around 200 to nitride a crank. closest place is Jax that I know of.

I would blame mechanic because you cant put a new bearing on a used crank that has minor thrust surface wear. it may look good to the eye. may even mike out within factory spec. if there is just .002 wear on crank it would still be well within spec. but the the surface does not wear evenly. it wears to original bearing. its like changing your brake pads without cutting rotors. not a great idea. in this case a very bad idea.

there are many other aspects to making a rebuild work. I dont have time to discuss them all.