Do afterburners use excessive amounts of fuel?
Yes the specific fuel consumption of the afterburner, lbs of fuel used per lb of thrust, is much higher than the core engine.  This is because the fuel is being added to a part of the engine where the air is less compressed, so the energy conversion is a lot less efficient. The bright orange flame coming out the tail pipe when in reheat is pretty much all that un-oxidized carbon fluorescing - wasted energy. See this article.

Fighters use reheat because all that extra power is available with minimal extra weight (the weight of the burner and additional fuel), but they can only exploit that power for short periods if they want to have decent range or endurance, so they are generally used for dashing somewhere, or to assist with maneuvering, or taking off with a heavy load.