If you elect to do plugs (mine were done at 28k, 37k is the max called for in the manual), buy the plugs yourself and have them do the labor. I just did that and the labor was $125 (1.5 hrs at $85/hour) and I paid ~$80 for the plugs on Amazon. ~$200 all in. $579 is outrageous even if you did buy the plugs from the dealer.
08 E90 | Jet Black / Black | 6MT
Mods: RAC RG63 w/ Pirelli Corsa 255F/285R | Akrapovic Evo Exhaust | OEM Blacklines
Winter Setup: OEM 18s w/ Nokian Hakkapeliita R 245/40-18