According to the National Park Service website, you can visit Acadia (and all other National Parks) for the low cost of NOTHING this Sunday (August 25th).

In celebration of the National Park Service's 103rd birthday, the admission charges (normally $30) are being waived for the day.

According to Wikipedia, the National Park Service was created by congress and signed by President Woodrow Wilson on August 25th, 1916.  The NPS' mission is to "promote and regulate the use of the Federal areas known as national parks, monuments, and reservations hereinafter specified by such means and measures as conform to the fundamental purpose of the said parks, monuments, and reservations, which purpose is to conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations".

Can't make it Sunday?  There are two other FREE days at our National Parks: National Public Lands Day (September 28th) and Veteran's Day (November 11th).