Although the Zimbabwe dollar is currently obsolete and no longer legal tender, one billion Zimbabwean dollars should be worth approximately $2,678,810 USD. The rate, according to XE.com, as of September 17, 2015, is $1 USD equating to 361.9 Zimbabwean dollars.
In June 2015, the Reserve Bank of Zimbabwe began to demonetize the Zimbabwean dollar, rendering it's official value at 0, with no worth. Zimbabwe plans to switch to the US dollar by the end of September 2015. While the value may be listed within currency exchanges, the current worth of the Zimbabwean dollar is non-existent, although the Zimbabwe Reserve is still accepting exchanges for some old bank notes as the USD becomes adopted as the official currency of Zimbabwe.