Well when I went to sell, I also had no idea about price. When I made ad I demanded 20.50 for it. But when I checked prices at Pakwheels, it opened my eyes. Also called people and one army officer told me his was sold for 21.25. Then I revised my price.
So keep an eye on Pakwheels. Filter results with model, city (Islamabad) and study demand. Compare with your mileage, condition etc. You will have a fair idea.
Remember Grande is once again in high demand. Waiting period is two months minimum and own 150,000 to 200,000 and WHT 150,000 non filer, so you should expect decent offers.