I looked up the value of the vehicle you mentioned on Kelley Blue Book (www.kbb.com). Without your exact zip code, trim level and other details, it is hard for me to give you an accurate value of the vehicle for trade in.
For a 2002 Mercury Sable GS Sedan 4 door with 100k milesand in FAIR condition (you can change the condition as well, but I always use this one):$2,750
Of course, take into account what you owe to know how much you can get for a trade-in on this vehicle. And remember I don't know what options are in it or if it is a higher trim level.
Check out kbb.com and do some more detailed searching to get a better estimate.