How necessary is a flush when replacing brake pads and rotors on front axle? on 2009 Pontiac Vibe

How much does a flush--while replacing brake pads & rotors--cost?

Asked by for the 2009 Pontiac Vibe
It is a good idea to flush your brake fluid every 24 months or 24,000 miles. If this applies, then it is a good idea. Brake fluid is a vegetable oil with some very special additives. It gets dirty in about 2 years and gets water in it, which compromises how well it works.
ThanKS FOR REPLY. What's a fair price on the brake fluid flush?