By saying "bravery/courage", we implicitly mean a hard task that we wouldn't do it in daily life, and we have to sacrifice something to get it - we are scared. It is, after all, just a cost-benefit analysis, but what make it distincts from a regular analysis (e.g. should I buy a car now) is that our attachments to what we are having is too much that our calculation/rationality is distorted.
To overcome that distortion/maintain our rational, we need to question ourself why we should detach to it, and why we should spend our time and energy to questioning ourself. Doing all of this requires cognitive efforts. So I guess we can give a rough function of braverity like this:
braverity = time and energy to question yourself and question why you should question yourself
Since time and energy are physical entities, with suitable equipment, we can objectively measure braverity I suppose.
Is this correct?
Related discussion: What is bravery? Is it even really possible to tell?