What is the most costly method of an authentication?

Biometrics

Biometrics These technologies are becoming more reliable, and they will become widely used over the next few years. Many companies use smart cards as their primary method of access control. Implementations have been limited in many applications because of the high cost associated with these technologies. Reference: Mike Pastore and Emmett Dulaney, Security+ Study Guide, 2nd Edition, Alameda, Sybex, 2004, p 265