Weren't the spartan hoplites/warriors noblemen or at least wealthy ciiti?zens And like every other noblemen of that time they like luxeries like wine, good food and entertainment. Such things caused high expanses i think. Then the red cloak, their armour and weapons were also of finest quality available. This would cause very high recruitment costs and to repair such weapons and the other gear would need a master smith or sth like that. I am sure there was a lot more that caused the high upkeep cost of spartan soldiers, but thats what i could think of.