coin+weight

OK, the basic calculation is scalable WRT specific gravity, so if you take this calculation, and multiply the output by the ratio of specific gravity of gold to (silver/copper/platinum), you'll get the right answer for that type of coin, assuming they're all the same size. Gold Piece to lb: Density(Au)=19.32 g/cc, 1lb=453.6g, so there's 23.48cc of Au per lb. For convenience, I'm switching this over to mm3. 23480 mm3/lb. From the PH, we measure the diameter of a gp as 31 mm, so radius 15.5, and π*r2 gives us 754.77 mm2 for the face area of a coin. I assumed a thickness of 3mm (about 1.5 nickels thick), giving 2264.3mm3 per coin, so.... 10.37 gp/lb. and just round that to 10, since we're just making up a thickness anyway...

So yeah, it really is 50 gp = 1 lb.