Advertisement is expensive on TV, because by selling the time for commercials is the channels source of income. If more and more people are watching the channel, than more valuable is the time...
The only people that can tell you how much are those company who are trying to sell their product. But as far as I know, those huge company like "apple" or "Pepsi" they all pay at least million of...
If you would take the cost of all networks and then average them, the cost may probably come around $1 million to $1.5 million for every episode. On free TV, there are 22 to 26 episodes per season....