I once heard a lightning bolt has about $35 in electricity in it, all the way from the cloud to ground. I don't know how accurate that was, it would be quite hard to accurately measure too.
Capturing an impulse of many kiloamps is nearly impossible- and capturing it with a tower 100 ft tall might only yield a few thousand volts drop, meaning only a small portion of the energy would be captured. You'd need a tower that went all the way into the clouds to capture it all without wasting it with all the light, heat, and noise of a bolt.
Towers sometimes pick up current from the electric fields generated (often called "sky power"). They actually want to short it out to keep it from damaging the tower's transmitter, rather than build up a voltage you can use. It's somewhat different than lightning though, and I don't think the amplitude is either strong or consistent.