08 impala ss for 07 shelby mustang with 4000 miles?

I have a 2008 chevy impala, i'm thinking about getting a 2007 shelby mustang. the dealer is taking my car which was bought new 5 months ago and paying it off and giving me the shelby for 45,900(that includes my rollover of 12000). my payments would be 800 a month for 5 yrs. would anyone consider that a good deal? on the brink of destruction here