My markup for most RF images is as follows:
500px $9.96 ($12.95 retail)
1,000px $30.00 ($39.00 retail)
2,000px $75.00 ($97.50 retail)
4,000px $150.00 ($195.00 retail) (I often leave this one blank, though, to steer customers to full resolution)
Full Resolution $220.00 ($286.00 retail)
Notice each step up represents 4X increase in resolution but only a 2X - 3X increase in price. More pixels per dollar the higher you go.
I haven't sold a single license yet, so I don't know if this approach is any good. But this is what it's worth to me to grant a royalty free license to my images. Too cheap a price I feel belittles my time and effort.