Do command cards cost anything to play?
Can you be a little more specific? In general terms, you can use a single card every turn (your turns AND your opponent turns, too) without cost unless the card specifies any other condition (as an example, the art and dark simmetry cards requires one action to be used)
By default, no. You just need the unit which uses the card (aka the chosen unit) must have the same number or more command icons in order to be allowed to use the card (like an officer who need a level of clearance to use particular tactics).
But some cards have specific conditions to be played (which is stipulated in italic on its game text). Some of them need to be played only after the movement of the chosen unit, other require the spending of 1 to 3 actions. But these cards are exceptions.