<p>It would be helpful to me if I knew in advance how much the <strong><em>empty</em></strong> filament spool weighs. </p>

<p>Not having emptied any spool yet, I can't contribute data points, but has anyone compiled a list of empty weights from various manufacturers and sizes?</p>

Yes, there is a table on
Reddit - Empty spool weights for estimating remaining filament
which suggests that the norm (in 2015) was between 170 and 330 grams for a 1 kg spool, superficially in a bimodal distribution clustered around the high and low points. Presumably these were for 3 mm filament, this post dating to before the popularity of 1.75 mm...
Manufacturer's have an incentive to reduce the mass of their spools, or even ship without spools, since the shipping cost will eat into their profit margin.