There have been a number of tools which help you access these volumes like Overture , however, this tool by WORDFINDER is excellent as it aggregates the results across various search engines.
While it is driven off the Overture keyword suggestion tool, it aggregates monthly regional monthly regional search volumes by market for Google, Yahoo!, MSN and ASK and then links the search volumes to the related global search results.
The tool currently assumes Google having 60.9% of the search traffic where Yahoo! having 22.3%, MSN 10.6% and ASK 4.3% of the entire search volume. Stats are based on a 2006 Hitwise survey.
Depending on your topic and your geographic location the search engines may have vastly different search volumes. The tool can only possibly offer approximations. It is sad that the geographies covered by the tool are limited.