The rotors don't have to be replaced every time. Just if they are below the minimum thickness or they are warped (you would feel vibration when you hit the brakes if they are warped). Our front ones lasted 60K miles and the rear are still original and in great shape with 72K miles.
Dollar wise, if they use Porsche brake pads, they run about $125 front and $110 rear for the pads and $16/wheel for new brake wear sensors. Three hours labor is what they will probably charge, but it should not take them more than 1 1/2 hours to do it so they may cut you a break if you ask (unless it's a dealer, then they won't give a break).
If you order your own brake pads next time, you can order them online for about $60 front and $45 rear for low dust Mintex Red Box. Great pads and nice, clean wheels.
If you have to get rotors, then it's much more. $150+ each for Porsche front and close to that for the rear. Actual labor is about an extra 15 minutes per wheel.
When the light comes on, you have about 3K-5K miles to get the pads replaced before they start to dig into the rotors so it's good that you're getting it done before the trip.
Hope it helps.
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