Depending on your needs and building type (residential vs commercial) Boxes can cost anywhere from $200-$1,000 each. The wide range in price will depend on if you want your Box to tie into your building's security system, size and other custom options.
Remember, these boxes will last upward 50 years. That's only $4 per year for a low end Knox Box. Replacing a new Exterior door is on average $1,000-$3,000, making a Knox Box purchase a much cheaper option than replacing doors that were forcibly entered.