£1500. The last car we traded in was put back up for sale initially with a £1700 markup, subsequently lowered to £1250. Don't know if it sold at that price or not, but I assume there or thereabouts. For more expensive cars, I assume a higher markup in nominal terms if not percentage.
If you think about it, unless they are fleecing you on the car you are buying, they need to be clearing a grand to make it worthwhile, when you factor in labour, valet, storage, cosmetic touch-up etc. Depending on the vehicle and dealer, it may go straight to auction anyway.
All you really need to care about is the delta between the car you are buying and the car you are selling, and whether that is in the right sort of ballpark compared to just selling your car elsewhere and buying the new car outright. Getting less for the trade-in is OK if you've got a market-leading deal on the car you are buying and the reduced hassle of trade-in. Invariably you will get low-balled and it can pay to shop around, if there are multiple options for the sort of car you are looking for. Last time out, we visited 3 franchised dealers (same manufacturer) and even after negotiation got three different offers.