Hi Trudy!
For me, it's absolutely worth the extra cost! We've done the Mickey's Very Merry Christmas Party the last 2 years, and 3 of the last 4. That being said, if you're not one of those "go-all-out-Christmas-spirit" types, then perhaps you won't be overly thrilled.
But it's always a highlight for our family, and Disney (particularly Main Street) is almost the perfect setting for a Disney celebration. Snow falls from the rooftops, the constant smell of baked goods fills the air, free cookies and hot cocoa is distributed throughout the Park.
And that doesn't include the character Christmas stories, and the knock-it-out-of-the-park Christmas parade, showcasing almost every Disney character I can think of, and, of course, ending with Santa Clause along with Mickey and Minnie.
So, if Christmas is your thing, it's definitely worth the added expense!