'Average'? There is no simple average.
It varies from person to person, with the type of connection, where the connection is made, and the 'emotional state' of the person.
If you just grab the leads from a resistance meter (or ohmmeter) then typically you get a reading around a million ohms, but even that is very variable. I tend to have dry skin, so for me the figure is several or even tens of megohms.
Actually the resistance between any two points INSIDE the human body is relatively low - maybe of the order of a few thousand ohms, or even less - because blood contains various ionic materials, such as sodium chloride, which conduct reasonably well. Most of the recorded resistance is due to the resistance of the skin, and that depends quite critically in the amount of moisture in it. Hence my reference to dry hands earlier.
The 'emotional state' of a person affects the amount of sweating, so in a lie detector test, one component of which is a skin resistance measurement, any anxiety by the subject that he might give himself away when awkward questions are asked tends to lead to increased sweating, even by a small amount, and this shows up on the test results. That's the theory anyway, and on unsophisticated subjects it tends to work well. On 'hardened criminals' it may well have no effect!