In California the fingerprint is not subject to more fees. The cost of a notarized signature is $10 per signature, with or without a fingerprint required. The purpose, among others, is to help avoid fraud where the usual identity might not be enough to prevent it, for example, John Smith Jr. may be trying to sell John Smith Sr.’s property.
It’s a great way for the government to get fingerprint samples from everyone. Just for their own protection, you know. It would never be used for any nefarious purpose by the government.
“Oh, and while you are at it, could you give a DNA sample as well. It’s for your own good.”