I was offered $7,000 twice for my 2007 Mazda 5 with 152,000km on it and an official Canadian Black Book trade in value of $3,600 low to $5,600 high. Realistically I think I could get $8,000, maybe $8,500 for it on the private market. Seeing how I get $490 PST credit on the $7,000 trade in, to me it wouldn't quite be worth the hassle to go after that $500 to $1,000.
BTW, I was also contemplating trading in my Mazda 5 because of fuel economy. I decided to buy an Ultra Gauge and alter my driving habits instead. I kid you not, reduced my gas consumption by 15%. Part of it is knowing real time how much gas you're using, the other part is doing something about it.