One way I measure how well a site is performing is by its return per image. This is calculated by dividing the monthly revenue by number of images at each individual site (ex: A site where you have 1000 images and earn $2,000 has a $2 RPI that month).  I'm curious to what others have. Here's mine for April. 

IS   $1.90
SS   $0.41
DT   $0.14
FT   $0.13
123RF   $0.12
StockXpert   $0.04
BigStock   $0.04
SV   $0.02
LO   $0.01
AB   $0.00
FP   $0.00