I just replaced my left side felt block for $35 and some sweat in the garage. Purchased from ECS Tuning (love those guys!) and it was easy to change out, just a few screws. I already had the car on jack stands and wheels off to bleed brakes but I would imagine you could do this without even jacking up the car.
Even for full undertray and BOTH felt blocks, $600 sounds way high.
2008 E92 AW 6MT ///eibach's///ARC-8///Muffler Delete///98k orig rod bearings