Well, the range of 500SLs of that age on sale will vary quite a bit, by spec, history, mileage and condition, so prices will vary too.
How "certain" a value do you need? If you just want to make sure you're putting a sensible value in the insurance form, then you can work out a figure from the prices shown for something that's most similar to yours.
If you think yours is unusually valuable, you can stick a higher price in. You may then be charged a higher premium, and if at some point you make a claim, you'll need to be able to justify it, through photos and documentation, or the company will ignore your figure, and use theirs.
Is it on classic* insurance? If so, you can normally agree the value, in advance, by filling out a form, and sending recent photos with it. The company can then decide whether or not to agree your figure, all in advance.
*Requires you to use it fairly lightly, have another car as "daily driver", and to be (usually) over 20 yrs old. My 1995 E220 Cabriolet is insured as a classic, comprehensive, unlimited mileage, guaranteed salvage buy-back, not garaged, and agreed value of £7,500. Adding it to my classic policy produced annual extra cost of £47.