I think it all depends on milage, condition of the car, and the individual dealership.
I allowed my dad to trade-in my P5 towards one of his RX-8s (since he traded in his truck to help out with my Miata), and he got
$13,880 for it! Of course, it was an 03.5 with under 20k miles, a few minor mods, and in immaculate condition, but we were both shocked, since we got the car for just under $16k brand new.
So basically $3000 depreciation for just over 3 years of ownership isn't bad.
Then again, our salesmen may very well have given us a little bonus, since we bought 2 cars from him previously.
Interesting note though, we were talking after I took my first test drive, and he saw that I had driven over in a P5, and he was saying how they don't usually get many Protege5s traded-in, saying people tend to keep them for a long time. I just smiled.