I'm actually looking to do the same thing, trade in my GT and get a used GT500. for the power goals I have it makes more sense for me to do that than to build the motor and supercharge my GT. even if a lose a little money on my GT I still come out ahead in the end to hit my performance goals.
I've been finding lots of GT500's on ebay with low miles right now with a few at dealers which will make it easier for the whole trade thing and financing. don't believe all that pricing crap, that's just dealer games. like 05 mustang said, don't try and haggle and argue with them, give them a price and leave your number, they'll call

I'm looking at all the GT500's listed at $38,000 - $40,000 with no bids, there's no way they're turning down high offers left and right when they're not going for that much on ebay. look at the big picture too, if they're giving you a lot for your trade it may not make sense to haggle a lower price on the GT500, but if they're giving you a low price on your trade get a lower price on the GT500.
you'll probably lose a little money on the GT, I probably will to, but why are you getting the GT500? if it's the same reasons as me then you want more power with the reliability of a built motor, fuel system, drivetrain, and upgraded brakes. I'm looking at the Kenne Bell stage 2 for my GT, that's $6,300 plus shipping and misc installing costs doing the labor myself. that basically maxes out the GT, the clutch, engine, drivetrain, fuel system, everything is at it's limit with a little safety margin. to go past that power level you need to build the motor, drivetrain, fuel system, tons of money if you want to safely go past 550whp. now the GT500 only needs a Kenne Bell stage 3 for $6,000 and you can have 800whp on race gas on stock everything, that's sick!!! I'm sure the whipple is the same way, I just mention the KB since that's what I'm looking at.
so for me it makes more sense to ditch my GT and get the GT500, even if it's a $10,000 difference between what I sell my GT for and what I get the GT500 I'm still ahead. that's mainly because I want to go to the higher power numbers. now if you'll be happy with 400-450whp then it probably doesn't make sense for you to get the GT500, just get an aftermarket supercharger kit or get Ford's kit if you want to keep your warranty. but if you're honest with yourself you know that power is addictive and you won't stay happy with 450whp for long
