Cost to install civic engine in 2005 mustang?
How are you going to do it? Make the Mustang front-wheel-drive? Or pay somebody to make up a one-off adapter to bolt a rear drive transmission to the engine? And wiring the Honda engine up to the Ford body? As Chandler Bing would say - "Can open... worms everywhere!"
EDIT: Well, Chevy-in-a-Mustang is pretty common, or maybe an older smallblock Ford, or an old straight six, but none of these would be emissions legal without a ton of work.