You can find deals here and there. I did actually pick up a complete nearly stock Automag a couple of weeks ago for $60 shipped. I've actually picked up a couple at this price along the way, but understand I am on the forums A LOT. Most Mags on Ebay get bid up pretty quickly and I would guess rarely sell for less than $100. Honestly, if you are looking for something specific, you might be better off just building what you want from scratch. Most times people buy a stock Mag and then replace the frame. Couple of weeks later, they decide to buy a Retro or X valve for it and then start looking at ULE bodies. Next thing you know, they have replaced everything on the marker excepting the rail, sear and a few screws. So in the end, not only did they buy a complete Mag, but they replaced nearly all of the parts and now have an almost complete second marker.
I also want to note in your post, the you mention Automags, MiniMags and MicroMags. These are three different bodies essentially. The Automag and MiniMag share everything except the body. The AM has the shorter SS body that ends at about the end of the rail, and the MiniMag has the SS body that is longer and has the "vent" slots milled in the front. The MM bodies typically sell for about $20-30 more than a standard AM body, thus the pricing difference between the two. MM's sell for $110+ usually.
The MicroMag is unibody that has a one piece rail and body. These bodies were produced by PTP. These typically go for quite a bit more than a standard AM or MM.
Here are pics of all three:
Automag with aftermarket frame and barrel:
I don't have a stock MiniMag, but here is a PumpMag with a Centerfeed MM body:
Nickel plated Micromag with aftermarket foregrip and Intelliframe: