If it's a 1979 it will not be fibreglass, which means it is worth a whole lot less than the steel version. A good fibreglass car will easily be worth €70k even with 80.000km on the clock.
As for the one you are looking at - well.... Low mileage examples of older high production Ferraris are very, very difficult to price. Let's approach it from the other side:
Forgetting about mileage, I think the most pristine 1979 GTB you can imagine should be worth a maximum of €60k, although I have never seen nor heard of any such sell for that price. Top of the market is more like €50k.
Now, let's introduce "your car". I think it is very, very unlikely that this car is in good mechanical condition. Normally, when cars have been standing for a long time, as is likely when cars have very low mileage ("yours" has done 156 kms per year!), hoses rot through or become brittle. Oil stands and sags and breaks down. Petrol in the tank has probably caused dirt to collect at the bottom. I, personally, would never start such a car until it had had a full inspection and replacement of all fluids, and better still, a proper light mechanical restoration. The bushes will in all likelihood be shod, and any rubber will be brick hard (for instance, window seals).
So, if we take these things away then the car is worth quite a lot less than a perfect running car. We only have to add what the low mileage is worth.
All these things will not be believed by the owners (or denied), who only look at the odometer and will want a ridiculous amount of money for something, which is, admittedly, truly rare. Is it worth €70k, €90k, €100k? More? Perhaps all these things, it all depends on the owners and the buyer to agree.
So purchases like this are not for the faint hearted, and IMHO are best left for people who already know exactly what they want. Because - even if you have the money and you want to restore the car, what do you do then? Are you happy NOT to drive the car? Because, you can never go back on mileage. Don't spend over the top for a low-mileage car if you then take it on a 3,000 mile trip of Europe.... If you spent 4 times the worth of a regular driving 308, do you still feel you got value for money? If you do, then you've made the right choice.
A low mileage car like this can be a wonderful purchase, for the right person who strives after perfection, and who is prepared to (and knows in advance) to spend a lot of money getting the car to be right.
But for most of us, things are not like that.
Bottom line - I think a 308 like this could cost you over €100k to buy and get running properly.
I hope this helps. And please remember - nothing in this sort of area is fixed. There will be people out there who disagree with me, and a large part of the cost is down to the choices that the owner makes. But if you want to do a classic Ferrari of low mileage justice, it is going to cost a lot of money. So just be aware of that before taking the plunge.
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