Hi Steve,

You've given us no details of a car so we assume this is quite a general question. And sadly the answer has to be quite general too as there's no science to it. You could be offered varying amounts for your car on a trade in because it will depend not only on its value, but also on how badly the dealer wants to make the deal and how much they do or do not want to have to prepare your car for sale. 

Take a look on Carzone.ie for very similar cars to yours for sale by traders and then assume you'll be offered several thousand less than that as a starting point.