Hi Grace,

There are too many variables at play here for us to be able to give you accurate figures - we could be wildly off even on our estimations. First up, every dealer is different and how much they offer you for your Astra on any given day depends on both how badly they do/do not want to resell your car and how badly they want to sell the car you are buying. Your Astra may be worth about €2,500 if you sold it privately, but a dealer will offer less as restocking and reselling it comes at a cost to them. 

And finally, 2018 Santa Fes vary massively in price depending on specification, age, mileage, etc. On Carzone right now I see over 20 examples, varying from about €28,000 to nearly €10,000 more than that. 

Hopefully that's of some small help to you in any case