Over here, R170s price does not seem to vary much on build date.
That is , 2003-2004 are not more expensive than 1999-2000.
It depends upon mileage, condition and model.
The lowest mileage SLK230 or SLK320 are still no more than 25% of there original new price.
One extreme example for a 2000 mile 230 asked 30%, but I don't know what it sold for.
Typically if they are 25-36,000 miles , 40-60,000km, people ask 16-21% of new price.
And We tend to over value used cars here too, because with no snow, salt or road grit, they don't rust and cosmetically usually look great. Early SLK used prices always look cheaper in US & UK , but maybe that's because the new price was lower too.
How much better is an 8000 mile car than a 25,000 mile car?, a little. Not 3 times better.
In some ways worse through lack of use, as El_Murcielago says.
The difference is about 2 years normal use, isn't it?
How much would I pay? Maybe 20-25% more than a 25,000 mile car, so maybe 25% of new price.
But personally I'd rather have one that had 2-3000 per year with steady use and attention all year, so it all depends. So maybe to me it's not worth more.
Either way it's a bargain compared to new price, you'll be enjoying a 70-80% discount for a great car.
When driving roof down there'll be,,,,One big smile for the car, onother for the price.
Buy it! You won't regret it.
Remember that the roof will not have been cycled very often and any long spell of no-use can let those cylinder o-rings decay with bad results,,,leaks and required repair.
So try to establish if it has just sat for for a year or two or more, or whether it really was driven every few weeks in summer. Average mileage being 650 per year, this car has sat around a lot.
Might be worth planning on replacing the o-rings anyway.
Lots of low milage ones about for comparison, finding some advertised examples will assist the discussion.