What if all contributors join the sites where you set your own price, delete all your images from other websites, and see what happens?
As long as there is a minimum price set on these web sites, eg. Can't sell a high res image for less than 5 euro or whatever, no-one can undercut their neighbour, and you will all get a nice return for your work.
The whole problem here is that you are letting the agency dictate the price, my advice would be to support the agencies that let you set your own price MUCH more than the agencies that don't, even if you don't take the radical move of deleting your images from them..
The unfortunate thing about this is that is takes an initial loss of income, but after that, more profit for you, as long as the majority follow suit.
How about committing suicide?
Think of the people who make a living off of this business... They would have to lose ALL their earnings from one day to the next (more or less) in order to upload only to sites that allow you to set the prices.
God knows what these agencies will accept of all the stuff those photographers already had accepted at other agencies. Not to speak of older material that is not up to reviewing standards anymore but still selling at the other agencies...
Then, and this is the most important point. Such set-your-own price agencies are still considerably small and barely have big marketing budgets. Who knows if they ever can take on the big boys in terms of advertising and getting the word out???
It's not just happening by itself that the buyers will be like: Hey this agency is fair to their photographers - I'm going to buy my stuff here from now on (although it's much more expensive), regardless if the image I really want is sold at Istock (that was just an example...).
It would be a nice thought if we lived in imagination land. Wake up.