The value of any coin largely depends on its condition. This one is in very very low grade and grades as Pr 2/Bc 1 (Poor/Basic for the obverse and reverse respectively). The reverse is almost entirely corroded and possibly not identifiable. The obverse is not much better.
The same coin in high grade (XF) could be worth, I suppose, more than 100€, but this is speculative until we know more (diameter and weight).
If this is indeed a sestertius of Nero, these are among the most sought after coins because the quality of the engraving, particularly the portraits, was at the top. But all the art is lost in this corroded one.
Now, don't worry, many people have Roman coins in this condition. I have given up a long time ago in trying to identify them here on Numista because they're not worth the time. If someone gave you 15€ for this, personally I would take it.
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