I think of return per image or RPI and return per download or RPD as a more accurate way to post stats. For example if you have 1000 images at a site and sell 20 images at 100$ each (dream on eh?) the RPD is 100$ and the RPI is 2$.
Sadly, both of those are down a lot across the industry. I certainly don't feel like microstock is worth my effort these days, I will still keyword and upload a few pics that I am taking anyway and put enough work in that I can deduct some expenses, but I certainly won't be putting a lot of work or expense into taking pics just for microstock.
Fortunately I don't need it to put food on my plate, or I might be considering working for a king or a clown, or try to find another option like prints or book covers.
I strongly suspect that footage is following the path of still images, probably at a faster pace.