Per PATC:
Effective immediately, the National Park Service will no longer charge an entrance fee for Manassas National Battlefield Park. An assessment of the fee program this year determined that collecting an entrance fee is no longer cost-effective. In addition to eliminating fees, Manassas will no longer offer the National Parks and Federal Recreational Lands passes or other interagency passes. These passes can be purchased at most federal recreation sites that collect entrance fees.
Story here : http://www.patc.net/PATC/News/Entrance_Fees_Abolished_at_Manassas_NBP.aspx