Here's an AP story that would benefit from a stylebook check. The world's oldest and largest newsgathering organization calls for multimillion, no hyphen. As a compound modifier, it should be multimillion-dollarhomes.
The AP Stylebook entry on multi- offers a general rule of no hyphens. Other than multimillion, examples include multimillionaire and multicolored.
The Webster's New World College Dictionary, which covers matters not settled by the AP Stylebook, goes with multicultural but calls for multi-tasking.