Details on how "slave price" was formed
We have some facts..1) A Trireme according to varius sources costed roughly 10 talants.
In the game the cost is 500 credits.So there is a difference of 50x the talant value.
2) A slave could buy his own freedom in periods of extreme danger for the nation with the cost of 5 mne which was the 1/12 of talant.
3)Now in a province of 60% slave population with a slavery building of max level you can get arround 50,000 credits from their sale right now in 4-5 turns (not every turn all profits combined).A province of 50% slave population will produce every turn arround 1200-1500 gold from this population according to some people (i never had so many slaves in my games so i dont know)
4) this is equal as 1000 talants or 100 Triremes (in ancient times not in game).So 60%population of a province (the number is hudge just imagine how many people this number is..) for 100 triremes or two years of the income of Athenian league (which was arround 600 talands a year).A number logical as we see in history.Spartans they could fund their whole expedition if they would sell Athenians as slaves after their defeat but they refused.This shows the big amounts of money earned from selling slaves.
5)Finaly roughly from selling you get the sum of money you would earn from them in 8 years of service if someone chose to keep them (with a 4tpy mod that is).Which also is logical.If you keep a slave you exploit his work for the rest of his life.So if you sell you need 8-10 years of work gain as a payment.
As it concludes from 4) and 5) this mods price is totaly logical and historical accurate.Nations used much less number of slave ransom to make up much larger sums of money in history.Slaves were the economic power of ancient world.Persia had 1,000,000 talands in its treasury during Alexanders time.THis equals to 50,000,000 credits of game money.