From Microsoft PressPass:

“Like the previous version of the product, BizTalk Server 2004 will be offered in four editions: Enterprise, Standard, Partner and Developer. The pricing for these editions is on par with the 2002 version, with ERP Open B pricing starting at $25,000 CPU for the Enterprise Edition, $7,000 CPU for the Standard Edition, $1,000 CPU for the Partner Edition and $750 per user for the Developer Edition, which can be used for development and testing purposes only.”

Prices quoted are in USD.