Look to see what a Stock Library would charge, Alamy
, for example.
However, keep in mind you have listed prices on your website, so they may be thinking your fee will be closer to those.
The other option is to simply ask them what their budget is or what sort of figure did they have in mind, for this type of use !!
Either way, just be sure to clear state what the use is, when you quote them a fee.
For example:To provide one image of [describe the image].
Image reference: [file number].
For non-exclusive use, for [their name] to use this image for:-
Media use: Desktop / screensaver only.
Period of use: 5 years (ends 28th February, 2017).
Territory of use: Worldwide.
Licence fee based on the above: $.............