You are just churning out the ideas, aren't you, Heavenfall?  Great work. 

Kingdom societies, it is generally assumed, inhibit a high level of personal freedom. Excess (unused) labour is to be considered a public good in such a state. I feel that the free rider problem http://en.wikipedia.org/wiki/Free_rider_problem can be applied to this. As the city grows, the excess labour becomes almost entirely emptied out by a volunteer station. Thus, the free time of the average citizen is reduced, even though not forced on the individual. This, in turn, means that the individual has less time to "care for his own garden", or provide assistance to his neighbours and so on. Even though it is only for volunteers, such a station will be seen as encroaching on the personal freedoms of the individual, especially if backed by a government. These things result in the Kingdom city becoming less appealing as a possible habitation location for those that the city is trying to attract from the wilds.

A volunteer station would only produce less benefits compared to a slave market in a society where there was a) no personal freedom and no culture to assist. A volunteer station would not work in an Empire city, and a slave market would not work in a Kingdom city. It is entirely possible that a Volunteer Station would produce MORE benefits than a slave market. In fact, that is the assumption that governs all the Kingdoms.

Ps. It's an arbitrary game value so that balance is maintained.

Heavenfall, if i may suggest:

I am going to mod your file this way,

Kingdom costs: (level 1 = .5, level 2 = 1 and so on)
Reduces Prestige in cities above level 1 (level 2 = -1, level 3 = -2 and so on)

Empire costs: (level 1 = 1, level 2 = 2 and so on)
Reduces Prestige in cities above level 1 (level 2 = -0.5, level 3 = -1 and so on)

I just don't like that all buildings seem to mirror each other. 

To keep it RP with your "Free Rider Problem" Kingdoms have reduced costs because volunteers are citizens and therefore feed into the economy. Slaves do not and therefore cost more, if they produce the same amount of work. A volunteer goes home and feeds himself, a slave sleeps in your [establishment] and eats the food you pay for. But, Empires have reduced prestige loss because they already have slave type buildings, therefore it's innate to their style.