I think you have to consider first and foremost what the quality of the image is and HOW MUCH the vendor would want to use it. If the image/s are exceptional then they command RM fees - these are quite high (I have put an example at the bottom of this for a flyer - website is similar price for both ed and ad rates - but of course if they are going to fork out 500 bucks they have a great need and your image need to be exceptional in their eyes/needs).
If the image is not exceptional then a better comparitor would be an RF price - after all virtually every stock agency has hundreds of RF underwater shots on its books. I have also put an example for RF which are based on size (standard RF pricing system) - some agencies sell cheaper some more expensive. Also keep in mind the photographer would normally only get 50% of this value. Also keep in mind that the buyer gets to choose between thousands of "good" images rather than limited to one photog - of course if your images are of their usual dives sites or off thier boats this could into play.
If the image is not upto much they could pic up a similar one from a micro agent for pennies (note: "not up to much" is my generalization - I dont like the micro model so am somewhat biased against it).
So IMO 50 buck for an RF licenced image for web use is not bad, you would get about 37 bucks through an agent and for an small RF in a flyer they would be using the same file (small size) so you wouldnt get anymore money (they have paid the RF fee already).
Of course if the image/s is exceptional then you would want to use an RM model. The RM model is very complicated but if you want to see one in action just go to a stock agency site and play with parameters on their calculators. Its interesting and you will find in some cases (uses) the RM price comes in below the RF (full size) price for stock esp for editorial usage.
hope this helps
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RF licence
High Resolution Image
$296.91
Commercial, Editorial or Personal Use
Model Release: No
Property Release: No
Image Size: 3200 x 4800 pixels *
10.67" x 16.00" @ 300 dpi
Medium Resolution Image
$186.91
Commercial, Editorial or Personal Use
Model Release: No
Property Release: No
Image Size: 1600 x 2400 pixels *
5.33" x 8.00" @ 300 dpi
Low Resolution Image
$76.91
Commercial, Editorial or Personal Use
Model Release: No
Property Release: No
Image Size: 427 x 640 pixels *
1.42" x 2.13" @ 300 dpi
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A RM licence for a flyer
This image is available for
Commercial, Editorial or Personal usage.
Category:
Brochures - Direct Marketing
Usage:
Brochures - Folders
Size:
Up to 1/4 Page
Circulation:
Up to 10,000
Duration of Use:
Up to 1 Year
Industry:
Territory:
Please select Worldwide*
or all territories below that best apply...
North America*
Africa*
Asia*
Australia New Zealand and Oceania*
Central America*
Europe*
South America*
492 USD
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* edited for typo