What is a fair charge for developing a "Proof-of-concept" website?

Basically, it depends on the elements of the site and what type of the content it will hold, i.e. if it is going to be static page then you cannot charge it as to develop web site with some custom CMS and shopping cart for example...

In the end it comes to that how much time you are going to spend on it (T) multiplied by your rate (P as price)

Total cost= T (time spent) x P ( your hourly/daily rate)

Also, you need to take into consideration if you are going to do any kind of maintenance,etc. And if you did some work at your work place check with your billing how much do they charge to the client and then deduct some 20-30 %..

I did like that :)

I charge by the hour no matter what the subject or intent is.

I have already come up with a final price for the site, including a monthly maintenance fee, but he would like to have something to show his partners.  It won't be fully functional.  Do you think a series of static pages that will replicate the functionality of the site is sufficient for that purpose?

I guess what I am trying to come to terms with is do I need to build the site to show functionality, or can I get away with something easier?

I know how to use Photoshop, and have a CC membership, but I have never done a mock-up using Photoshop, as I have seen others do.  Would even basic Photoshop mock-ups be adequate for the "proof" stage of the site?

As you said, it is going to be "show&tell" then make them presentation with some images of what it is going to be.

This is exactly the type of thing I was looking for.  I realize that my heading may have been a bit misleading, but this definitely will help me in this and future projects.  A big thank you to all who participated for your comments.