I have purchased CDs on behalf of my employer, and I can tell you that when dealing with professional quality vendors, a content-to-cost ratio of 100 images : $500 is not uncommon at all. Case in point,
Getty.
Comstock has
multi-disc bundles and other special offers that are a little less shocking.
Thanks to whoever posted that link to
sxc.hu, it looks promising.
IStockPhoto is another great free resource that I use personally.