Biosafety prevents or delays the farm from being infected. It doesn't lessen the impact once it happens. One bird can infect all the others in hours even when they are in separate buildings just from spreading on clothing. If they are infected they will die.

If a small farm gets an infected bird, they can't just raise the prices of their eggs. Those eggs will just disappear from the stores because all the birds are dead. If they are doing rigorous isolation, like hiring totally separate people taking care of completely isolated flocks, that should increase their prices. Farms that are spending more money on isolation and chosing not to increase prices are still at high risk- they just haven't been unlucky yet.