The car industry makes money on effuel reveiws any margin of profit. If you can find out how much the dealership is generally buying their cars for, it will make it much easier to find their bottom line. This means you need to do some research on your local car market.
Don't let the salesperson know you have a trade in until you have a firm sales price in hand. Some dealers will inflate the price if they think you're trading in a car, so they can offer you a lot for the trade in, which is, then balanced by the new price they give on the car they're selling.