Somewhere in the recent-ish past, I had seen that someone used a garbage disposal as a hammer mill for cracking poultry feed. I don't know how well it worked in the long run, but they reported being happy with it at the time.
It might have been in "Farm Show Magazine", but I have only the vaguest recollections - just stashed it away in the mental compost heap. I don't remember if the correspondent had rigged up interchangeable screens to control the size of particles exiting the unit, and I don't remember just what they were grinding (though I wouldn't think oily seeds or very fibrous materials would be a good match). On the other hand, here's someone grinding up fruit with a garbage disposal, to feed the chickens, so maybe oily seeds wouldn't be a problem:
It looks like that model specified is about $170 US on Amazon. I'm not sure that's cheaper than a used hammer mill from an estate sale - if you can find one, that is.