There are commonly two types available.

Ones that fits to the servo and ones that fit to the bulkhead.

The big advantage to using a balance bar (or pedal box) to operate the brake system is that the master cylinder sizes can be tailored to suit the front to rear balance and the caliper piston diameters, if you have fitted bigger brakes. Final balance adjustments can then be carried out using the balance bar either on the bar itself, or remotely via a dash mounted cable adjuster.

A further advantage is that in most instances it does away with the need to fit proportioning valves, to control or reduce the rear brake pressure.

Cost depends on what type you get, and mods and ease of fitting depend on where you get it from.