As the guys said you need to go show your face to engage a dealer properly, other than that they wont go all out with you on the phone
Its a bit of a "how long is a piece of string" question
It depends how the car is priced compared to other similar ones .
Is its condition good or bad and is big on miles or not
e.g. if the car is 3500 , check carzone and see what the cheapest and most expensive is, for cars of the same year, engine size and spec
then find ones with similar milage to your own and pretty much thats your price to aim for.
A dealer is as aware of carzone as the private buyer and right now cars that are selling are the ones that are at the lowest in price compared with similar ones around the country
Do remember a dealer will be a bit more expensive than private punter and they will supply the car serviced with a 3 month warranty for a car of that value.
If you want pm me and i will tell you exactly what its worth
Regards
Brian