Go for a good nut(like a 'Earvana' or 'LSR roller nut').
Have it setup after installation(by a guitar tech or yourself)

the nut replacment(provided that you already have the nut you want installed)will cost anywhere from $5 to $20.
With a complete setup(fret crowning,truss rod adjustment,hardware check,intonation e.c.t)it will run you from around $20 to $60.

Dont overpay though...check around your local music stores before you choose a shop tech.