The Post finds out that if sold, the inflated balloon of artist Jeff Koons' Rabbit, which made its debut appearance in the 81st Macy's Thanksgiving Day Parade on Thursday, would go for $5 million. The balloon isn't for sale, but the parade is so happy with the balloon that it's interested in ideas from other artists. Um, formaldehyde Damien shark, anyone? Or how about a Takashi Murakami Mr. Pointy balloon? Dan Flavin light balloon?......