I was also curious about cost so I checked on-line price of new rotors. $60 each for Brembo fronts and $35 each for ATE rears. (300SL) It costs very little more for new vs turning. I realize that a shop needs to mark-up because they have "overhead" which isn't covered strickly by the per hour labor rate. I've never had good luck with turned rotors: tried it a couple times on several vehicles and always ended up buying new anyhow. If they aren't warped or grooved and aren't below minimum safe thickness, I'd leave them alone. If you are concerned about residue, clean with very fine steel wool then wash with brake cleaner spray.

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