I've done it a couple of times myself and know a couple of big outfits that are doing it every year. I don't know if it's the same everywhere, but it seems to be a pretty convoluted formula for what sort of discount you get. I've heard as high as 15% on really big deals ( 5 million or so) and I've heard of about 4% for say 3 new tractors. You just have to get with your salesman and let him see what he can do. In my experience it's nothing to get excited about unless you're trading at least 6 units at one time.