I was once offered about $20-25 per image to supply a new library with images. I was fine with that and hammered off a pile of table-top images for them. They then got back to me and said they had decided the price was too high, they would pay about $10 per image and they didn't mean simple images, they wanted top-end production with all the props etc.
Naturally, they could eff off on that. I just dumped what I had shot on the micros. But $20 per image would represent $200-300 for a days work, after costs. In my view, that is fine as a day rate for simple home-based work. Talking about not parting with copyright because the images can fetch hundreds or thousands of dollars is just BS - it is the average return that matters, not what your best-seller makes.
There is nothing wrong with being paid to work on a contract where your employer retains the copyright of the work you produce. Lots of people work on that basis. Producing hundreds of shots so that someone can cherry-pick one or two for $20 would be a different matter.
To look at it another way, if you want me to produce stuff in bulk for you and give you the copyright I would expect you to pay the equivalent of my probable commissions on that work for the first year or two. Then, after a couple of years, you would start showing a profit - much sooner if you are actually the agency and are good at your marketing. I think that is fair to both parties.