It should be done at each brake pad change. I don’t see a need to do it sooner than that unless your brakes last forever. I just did mine, brake pads and brake fluid flush after 5 years and 138k miles.
f you didn’t have the brake fluid flushed when you last had your pads replaced, you should have. If it has been a while since the pads have been replaced, maybe you should have them checked.
repair shoppe? No wonder its $70.