It seems to me that bullets are much cheaper than lethal injections OR electric chairs. For that matter, rope is even cheaper and can be used over and over again.
Actually, re-using a hanging rope isn't a very good idea -- it can result in some gruesome consequences.
The problem is that re-using the rope will eventually destroy its elasticity, taking out the natural "slack".
An inelastic rope can result in the condemned dropping thru the gallows floor...and popping the head right off the body.
This happened at least once in a public execution -- in Clayton, NM. "Black Jack" Ketchum, a prominent local outlaw was tried. convicted and sentenced to hang for train robbery in 1899. The gallows was constructed and tested the day before the execution, using a bag of cement to simulate the weight of a human body.
After testing, the rope was left extended thru the trap door overight, with the bag of cement still hanging on the end. As a result, the rope lost all its elasticity...and when "Black Jack" reached the end of his rope, his head was separated from his body.
This happened in full view of the audience, as the gallows was wide open underneath.
The complete story of Black Jack Ketchum and his gruesome execution may be found here, complete with photographs