aorund $5 over spot is a pretty good guide, though if they become hard to come by and no longer minted, this margin will increase...
You can get predecimal aussie silver for SPOT +- 1% and its probably 'cheapest' silver around. And id suggest for everyone to have at least 2-3 oz of threepences, sixpences, florins and shillings. They are SPOT +- 1%, they are liquid, they have circulated as money in this country too.