Hi there Fatford62!
As you can see, it varies - buying from a dealer? Ebay? Perth Mint? A fellow stacker?
What is reasonable is probably 6-10% (that's my historical variance for a 24-bar history - both from site sponsors and the sales thread).
I think they were all Perth Mint bars - not that the should be any difference, just that the Southern Cross/Silver Stackers bars are, well ... "pretty" and the Perth Mint ones are ... bars.
Hope that kicks the thread off - I'm sure there will be others who say from time to time you could buy down at 3% and snag a bargain - seen some great prices on here recently!