IMO, the Bersa Thunder 9 and Bersa Thunder 9 compact is a heck of a good pistol. Not JUST a good value - it's an INCREDIBLE value. It's a
darn fine pistol in its own right. Very well made, good features, pretty good ergos, reliable, high capacity, accurate, etc.
If you're going used, the old Star guns are universally good (Firestar, Modelo B) for the money, although not very high capacity.
You can get a drop in 9mm barrel for a CZ52 for about $75-$100, and then the CZ52 itself for $150-$175, and you've got yourself a nice well made 9mm (albeit very large, and single stack).
The FEGs, Arcuses, and Witness pistols are good, too - coming in at the $350 plus or minus range.
The Daewoo tri-action guns are a tad bit more, but also an excellent value.
There are quite a few used police trade in guns at cdnninvestments.com , but I don't look at their catalog anymore since they decided to become PC and eliminate the pistol prices without having to call them - what do they want me to do - spend 8 hours on the phone with their staff: it would be like Borat in the cheese aisle: "so what's the price on THIS pistol? uh-huh - and what's the price on THIS pistol? yeah, and what's the....."
You can get the best values in 9mm pistols, because they are the most popular worldwide; being the most popular, there are the most different gun models made, which gives rise to stiff competition, which keeps prices lower.