United StatesWhat to charge for unlimited use of one portrait?
Depending on the usages, geographic area of usage (ie.. North America, US Market, Regional market US, or world wide usage and the length of time they want to use it, it could range from a couple of hundred to several thousand. Check it out on some of the Stock Agency calculators.
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Also it sounds like you are not so sure on your wording of the license. Unlimited what? Time? Places it can be shown? Use in what kind of media? All those questions and more are a factor in the price.
Don't do it. Own the license. Of course this depends on what you arranged with the sitter in terms of model release!
I say this because the sitter will simply supply your image as a free image whenever any editorial is generated. You will see your image being used in major magazines etc with nom payment to you.
I photographed an author/public figure for an organization for which he acts as a consultant. He is now asking to buy the rights to use one of the images for his personal publicity and promotion. The agreement he would like is unlimited with the exception of advertising.