If you are one of the many car owners who gets a sinking feeling when a salesman tells you how much he (and it is almost invariably a he) is charging you for a trade-in, then you need to know a bit more about how cars are actually valued after a certain period of ownership.
We all know that the moment you drive your new pride and joy off the forecourt it immediately becomes worth a good deal less, but that figure might be a lot lower then we ever thought.
There are several factors that affect second-hand values, which are reflected in this month's Merrion Fleet survey for Motors.
Looking at the whole-life costs of the top sellers in the medium segment - in both petrol and diesel versions - we can see some wide discrepancies between what a car costs new and what it is worth after three years and an average of 24,000 kilometres a year.
For the purpose of these calculations, the leasing cost is generally the same as the average buyer will be paying for a new car with borrowed finance - and even the rate of that finance may be artificially low.
According to Merrion Fleet's remarketing manager, Graham Armstrong, who heads up a crystal ball-gazing committee whose job it is to predict what a car is worth after three years, there are several factors at play.
"There is the question of the number of imports of cars, which is increasing, and the numbers of these imports that sell. This in turn determines how many will be coming onto the second-hand market in three years - as well as other factors, such as the stage in its lifecycle a car is at.
"It will be worth a lot less if it is approaching the end of its model life. Other factors having a bearing include sales trends, reliability, what people are prepared to pay and what their lifestyle choices are going to be." Merrion specialises in whole-life cost analysis that passes over the heads of most of us. The company has previously noted how few people know what rate they are borrowing money at.
"Neither does the average person look at what the new car of their choice is currently worth after three years," says David Wilkinson, Merrion's sales director.
You can see from the table what value Merrion puts on a car - in trade value terms - after three years and 24,000 kilometres a year. The big names: Toyota, Volkswagen and Ford do best with the worst performers being Renault and Citroën. The VW Passat petrol version is estimated to be worth €13,800 after three years, while the equivalent Renault Laguna is worth only €7,475.
In diesel versions of the Renault Laguna and Citroën C5, the drop in value is some two thirds.
Companies like Opel argue that analysis such as this does not take into account the level of discount offered to the fleet and general customer "up front", but it seems fair to counter that not everyone gets the same level of discount.
There are trends to buying, says Wilkinson. For example, when the Alfa 156 came out "everyone wanted one" and the same was true of the Laguna when it came out, but the big and reputable names tend to remain up there in terms of popularity.
"We are talking four-door saloon territory here and the public tend to be set in their ways, but the new Mondeo should shake things up considerably."
And what of the diesel trend here? Armstrong sees it as a growth area, and points to the increasing value of second-hand diesels when compared to their petrol counterparts.
As a member of the buying public you may not be able to pick up cars for the kind of prices they change hands for in the trade but at least you now have a much clearer understanding of how other people value your car.
The data above is based on prices at the end of October, estimated on a car that's owned for three years, fully serviced and averaging 24,000km per year.
The final cost per kilometre would be considered as a guide for the difference between various models.
Similar to official combined fuel consumption figures, they may differ between owners but act as a comparison between models when official fuel economy and service charges are used.
The leasing cost per month is a guide to the monthly depreciation owners can expect to incur over the first three years of ownership of these models.
Total Service Cost includes road tax, servicing (in line with the manufacturer specifications/at least once a year), any estimate based on past experience for other vehicle repairs required outside of routine servicing, a courtesy vehicle for all service and repair work, 24-hour roadside assistance, and tyres.
Total Service Cost does not include - insurance or the cost of any repair work required due to accidental damage or driver abuse.