Trade in my car or keep it?

I have a 2003 G35 Coupe in pretty good condition (has some dings from street parking). I still owe $18,000 on it (I leased, than purchased which I realize is a ripoff) with monthly payments of $420/month.

I'm interested in getting a new but cheaper car to lease- like a Jetta or Altima- since there are some cheap lease deals out there (about $3,000 down, $200/month) and I get tired of cars after a few years.

Will I be totally ripped off if I trade the Infiniti in for a new car? I'm not sure how it works since I still owe a lot on the Infiniti. I'm interested in doing this because I don't want to continue paying $420 a month for a car that I don't really want anymore.

Thanks for any advice!!