[@Ryougu] Sorry when I wrote it I meant the 1 less mana was for the Piledriver still paid the full for dragon fodder that's why I had the mana count in brackets.

4 mana
1 spent - goblin Piledriver because of ability of Warchief
2 spent - dragon fodder
7 goblins in total
1 spent - Brightstone gem Gain 7 mana
3 spent - Hordeling outburst
1 spent - goblin Piledriver
2 spent - goblin chieften
1 spent - goblin grenade 

Where did my math go wrong?