Spray some brake cleaner in there and see if that helps.
$700 for just the rear sounds like a deep effing. Are you a cca member? That gets you a discount on dealer service/parts.
You can get rotors and pads from turner for like $400 for all corners. How many miles? If you only use the car for street driving maybe you can just have the rotors turned.
330i ~ BMW Perf. Susp. ~ M3 Front susp ~ BMW Perf Exhaust ~ 3.38 lsd ~ SF71 Alf CSL reps ~ Pilot Super Sports ~ AA Tune ~ 335is front brakes ~ over 155,000 miles hardly broken in! WTB: e93 M3 Front Sway Bar