Its a hard nut to crack,that equation.
Take the time to gauge the request,and the person/organisation,and what they want to use,and how they want to use the image.
A government department using a wildlife image,is going to be different to a aerospace company wanting to promote a aircraft,and so on.Each request needs to be looked at in a different way.By getting the full details of how the usage request is going to be used,will help with working out a fee.
Most request for larger firms,around $100 to $250 per request and go up from there.Also be aware of time frames-is it going to be a never ending use for $100?
It feels a tad uncomfortable,when trying to work all this out,when it first happens-we are doing something we like,and then hey-you have this image people want to pay you for.
But realistically,you have traveled a long road,to get that image (and payed a heap in gear),so its not unreasonable to get some pay for a few here and there,if you have been approached-its not like your out there pushing images for sale.
Once you have a few small sales,and maybe a few photo comp wins,you can find yourself going further ahead in the field.
This year,I have had a national magazine cover ($250) a few small articles,and images used in govt ($1200) and a few other odd photography jobs,so it all adds up,and helps pay for gear.We are always buying something for our photography! Even now-I have just spent $50 on ebay on diving lanyards-for hanging on to bits while wondering around the field!
Good luck
