7/7 is worth about 6.5 mana.
This is a bit of a side discussion, but what would War Golem have to cost to see play?
7/7 for 6 mana. It's better than Boulderfist Ogre, but that doesn't see play at all. The stats still aren't worth what legendary 6 mana cards can do like Sylvanus & Thaurissan. I think Savannah Highmane & Fire Elemental are still better also.
7/7 for 5 mana. There are conditional cards close to this stat amount.
Bolvar can easily have higher stats if drawn at the right time, and often is played at 6/7 to dodge bgh, but he isn't used at all.
Druid of the Fang is 7/7 in a beast deck, but it's not strong enough to form a beast deck around.
Upgraded Repair both is 5/5+4 in stats but not played.
Vol'jin can gain more than 6 health, and it acts as damage to an enemy.
7/7 for 4 mana seems insane.
Edit:
What if Dr. Boom is actually the balanced card? It's everything else that is just too weak against low cost high damage minions. Maybe Dr. Booms stat line when combined together is what you should get for 7 mana. A 7 mana 9/9 battlecry gain 2 to 8 attack.