The Antique Map Price Record is a CD-ROM-based reference tool that bills itself as more than just a listing of map prices (at auction, for example); it also contains reference images and bibilographical material, according to the publisher, who also maintains a list of antique map dealers on the site. Now in its 21st edition (vols. 1-16 were in book form), the CD-ROM sells for $149; updates to previous editions are $50. (Note that Mac support is touch and go: the Mac version is an OS 9/Classic app that will not work on Intel-based Macs.) I imagine that map collectors rely on this, or something like it, rather heavily. Via Map the Universe.