2015 Prius V 53k error code c1246

I'm at the dealership and they're quoting me an $1800 fix. Their notes say: "Health test code C1246 #203, ckd stop lamp switch install and operation okay, read freeze frame data and regulator pressure sensor voltage was out of spec, cleared code unable to duplicate, needs brake booster/ master cylinder and retest."

Am I just screwed? is $1800 going to make this problem go away? It's more than a little frustrating that this code isn't covered by the brake booster recall...