A few months ago I asked aprofessional nature photographer in Calgary a question along the same lines. Here's what he said:
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span style="font-family: 'Arial','sans-serif'; color: blue; font-size: 10pt; mso-fareast-font-family: 'Times New Roman';"]I usually charge $25/image if it's screen resolution for presentations and the web, for print I usually charge $50 for 9 x 6 x 300 dpi, and the price<span style="mso-fareast-font-family: 'Times New Roman';"]<o></o>
<p class="MsoNormal" style="margin: 0cm 0cm 0pt;"]<span style="font-family: 'Arial','sans-serif'; color: blue; font-size: 10pt; mso-fareast-font-family: 'Times New Roman';"]goes higher for larger sizes or for print. For books they usually pay me about $200-$300/image. For a powerpoint presentation - I would probably charge around $25/image.If you think you can get more thenby all means - if they come back and say they can't afford it, sometimes you can ask them what amount could they afford and you decide how many images you will sell them for the amount they have in their budget. Usually the more images they purchase the cheaper the price per image. If they only need one image,then I charge more mostly for the time to process and deliver the image.If they intend to use the images for print as well, they you could charge more. <span style="mso-fareast-font-family: 'Times New Roman';"]<o></o>