Wow, these are going for a lot on eBay! $200-$300.Ptolus is, quite simply, the most amazing and complete fantasy city reference book I have ever seen. The production quality is gorgeous, the ease of use is impressive, and the descriptions of the city are immersive. It truly is a complete city-in-a-book, with more potential storylines and plot-hooks than you can shake a stick at. The cost when I purchased it was $100, and I can honestly say I believe it was worth every penny.
From what was said here, Freeport seems like the best choice for what you intend. Do you mind the Cthulhu-mythos vibe?
I third the recommendation for Ptolus:I was going to suggest the Greyhawk boxed set. Tons of possibilities for urban adventures.
What makes Ptolus good, [MENTION=913]Schmoe[/MENTION]?