Dominic, the Michel (at least the 2013 edition I have) does not individually price the imperforate stamps of this set, but to give you an idea of relative value, the catalog value of the full set of 18 values perforated is €20.00, while the imperforate set is €40.00.
And the catalog value for that particular stamp perforated is €0.20, so perhaps you could impute a value of €0.40 for the imperforate stamp.
When I look at this stamp magnified like it is, the perimeter edges don't appear to be cut cleanly and the vertical sides are of unequal lengths. When I put a ruler against the stamp's left vertical edge, there is a ragged, uneven line. But then if one would look at me in high definition .......... and I, too, seldom give a straight line
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And the catalog value for that particular stamp perforated is €0.20, so perhaps you could impute a value of €0.40 for the imperforate stamp.