this gives you an idea of the cost, new (but not the availability, though it's presumably available):
Alfa Romeo Ersatzteile - italia Ricambi - Bremskraftregler (Hinterachse) Alfa 145 / 146 / 155 320173
they are part of the alfisti.net group, so their service is excellent.
it's a pretty cool setup, as it automatically adjusts the rear brake bias. the lever is connected to the rear antiroll bar. if for example you have lot of luggage or passengers, the rear suspension will be compressed; this acts on the brake bias and increases the braking force to the rear brakes.
likewise, with any braking, the trailing arm rear suspension means the rear suspension also compresses/squats down. the harder you brake, the harder the rear squats, and therefore there's an increase in rear brake bias and more braking.
if it's seized, then the rear brakes are 'set'. it's still safe, it still works, but just not as good as it could be. having said this, i don't think i've found the perfect setting (cos it's adjustable). it's tricky when the rear is lowered; at the moment, the rear isn't biting as hard as it was before (since i've lowered the car more and adjusted it). so i'm going to adjust it again and increase the rear braking. but of course, i could go too far, which might make the rear unstable particularly in wet weather.
changing it should not be too hard. probably just messy, cos the fluid would need to be drained.