You may have noticed I have a bee in my bonnet about the subject

The thing to remember is you're not simply selling on a piece of metal at a profit - you're actually adding quite a lot of value to it. There are ancillary costs alongside it - packaging, hallmarking, consumables (e.g. polishing, burrs etc.); then there's the cost of selling it: If it's sold in a gallery, then the cost of sale has to include lighting, wages, insurance and everything else.

Your £10 bit of silver has gone through a number of transformations already, each with attendant costs. The ore it was mined from probably didn't cost very much, but everyone in the chain has to be paid.

As for assay costs - while I hand-carried my last packet in to London (reduced the postage cost, increased my time costs - but there were other things I needed to do too), I put it through as Early Hall and paid just under £2 per piece including return shipping. I don't think that's excessive, but I appreciate that the costs on smaller packets can bump things up a lot.