I'd pay $100 for a complete re-do of Greyhawk. I'd also pay for a *complete* re-do of the Forgotten Realms, including Zakhara, Kara-Tur and Maztica.
The book would *have* to have the following (whether Greyhawk or Forgotten Realms):
1. Detail on each region to at least the level of detail in the FR region books.
2. Detail on all of the races and languages - to at least the level of detail in Thomas Costa's "Speaking in Tongues" Dragon Magazine article - and appropriate real-world equivalents for language and culture.
3. Detail on all of the various monetary systems and coinages, including "standard" exchange rates.
4. Detailed trade and economic information (imports and exports by region and city, major trade routes, taxes and tariffs, etc.)
5. A detailed full-color map of the entire world and various smaller scale maps for selected regions and cities, including climate maps, and also lat/long information and astronomical data.
6. A view of the night sky from both northern and southern hemispheres and information on the inhabitable moons and planets in the solar system (Spelljammer information).
7. A set of updated fonts for the various alphabets in use (such as Espruar, Thorass, Chondathan, Draconic, etc. for FR) - either as an included CD or downloadable.
8. If you wanna make it leather-bound with a book-mark, that'd be even better.