They cost 500g upfront and require 300g in upkeep per day. They will be a random Base Species with one random genetic trait and a Max Level of 10. Getting them injured or killed will reduce relations with the King. They can't be used for Breeding, Fusion or rituals either.
Sounds like a lot of downsides, but they are good early game for a quick boost to your party's fighting capabilities, and when you turn them in you get 250g for every level they gained. Do it before the day rolls over, and you don't have to pay that upkeep either.
Also see EliteSoldier