The release of the Gen4 Glocks didn't do much to lower the value of used Gen3 guns in my part of the country, but part of that could be the recent new-gun price increases and generally high demand for handguns. I traded-in a used Gen3 Glock 22 in .40 to get a new Gen4 of the same model, and they gave me more in trade for the Gen3 than I paid for it 5-6 years ago. That same used G22 is now on the shelf for $450 (fairly minty shape), and the new Gen3s are $550, with the Gen4s hovering around $579-$599.
You can find beat-up used police G22s for $339-$399 online, but after you pay shipping and transfer fees, you're usually over $400-$450, so I'd say that's probably the floor for retail pricing, at least in my area.
Here are a few links to places that are selling used police trade-in G22s: