I have worked for a client for about 20 years, they have asked me if they can buy lifetime rights to all my images were talking around 35 to 40,000 images how do I possibly put a price on the images many of the images are full of very famous people . Thanks
As others have mentioned you need to consider exactly what rights they want?
Do they want exclusivity? This would prevent you from any other use of, or profit from, the images.
Do they want the right to assign and/or sub license? In others words, in addition to this client being able to use the images, do they want to be able to sell/license/give-away the images to others? This would allow them to compete with you if you try to license an image to others?
Will them indemnify you for any claims/issues that arise from their use of the images. For instance, if they use an image for an HIV advertising campaign, it might imply that the model had HIV. Such use typically requires explicit permission in the model release. A general "for any legal purpose" release may not be enough. If the model sues, will they protect you?
Do they want to be able use the images for any purpose, or are they willing to me limited to a fixed set of enumerated uses.
Once you understand the extent of their license, you want to ask yourself what your costs will be. Are these images that you would otherwise make money on? Perhaps from this client, perhaps from others? If so, then you may want the licensing fee to be at least as much as the potential income you will lose.
Does a licensing agreement help or hurt your reputation? Will it make other clients (or even this client) more likely to use you in the future because they view you as being easy to deal with?
If the agreement requires them to credit you, will it bring you more clients? (I had some photos in the local paper the other day. I doubt my photo credit will bring me additional business).
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From a business perspective, the evaluation is simple. What is our expected future income from the image library? How much will that be reduced if you grant a license to the client? If the license fee is more than the lost revenue, then you come out ahead.
You've reduced your risk. You get a known price today, instead of potential future income. That fixed price may end up being more than you otherwise would have gotten, or less.