How much to charge for a digital file of a Landscape image?

[glennb]

Hi all, A friend of mine has a work colleague who wants to purchase one of my landscape pictures but he wants it in digital form so he can print it onto a large canvas for home. I have know idea how much to charge or if I should sell it in digital form at all. Is anybody able to give a range in price for something like this just to give me guide line to work with and also, should I sell it with a watermark and conditions eg. No reproduction of the image... If so how would I go about this? I do have copyright data in the image, is this enough? but I do know this cannot stop the misuse of the image if he chooses.
Cheers