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I would be surprised if spending £20 or so to get the weight polished added £20 to its sale price: in my experience these basic Mdina pieces are 'charity shop/car boot' items, which sell for single figure prices (ie £3 - £8) when in very good condition. So getting it polished would probably be equivalent to tearing up a £20 note. But if you really like it, and want it restored to perfection, it is not overly expensive to do it.
In general, I don't think it is worth repolishing the majority of Mdina, Strathearn, Murano, Chinese, or any low value weights, just in order to make them saleable: the figures don't stack up.
Alan