In the past few months I have gotten several requests to use my photos for different applications. I would prefer to sell an actual print print because the pricing is simple. I total up my costs and add a percentage. Easy.
But selling the use of a digital image is a different matter. I need to consider the resolution (is it to be used on a website or in a brochure?), as well as the number of times the image will be used. If the image is to be used to sell a product, I need to be compensated as well. (However, I am usually happy to donate images to non-profit organizations.)
But there are so many people/companies out there who want photographers to believe the only compensation they need is credit, not cash in the bank. Too many photographers take the bait and leave the rest of us, who would like to at least recoup the cost of that new lens, out in the cold.
So you take your chances, you win some and you lose some. But in the process you learn. There is no free lunch because there are no free lenses out there.