A true balanced design is a differential design, with both positive and negative legs going through mirror image gain stages, output stages, etc. To do balanced right is expensive, because you require duplicate circuits. A regular preamplifier has an input, buffer, gain and output stages, but only for the positive leg of the input. The balanced preamp will have identical circuits for both the positive and negative legs.
As stated previously, it is more expensive because of the additional circuitry. The benefits are basically a blacker background, but are really only noticeable when running fully balanced, from source through amplifier. If either the source, preamp or amp are not fully differential designs, then you will not be listening to 'balanced' output.
John