I find it a useful rule of thumb to assume that a coin's value decreases by 50% for each full grade it drops.
For example if the coin is listed at $10 in EF then it's reasonable to guess that it would be worth $5 in VF. For half grades, e.g. VF+ and AU just split the difference. So $7.50 in the example above and in the example you quoted, $29 ($8 + $21) in AU.
A couple of words of caution. If you are assigning a half grade to a coin then be 100% sure it's justified. At a guess I'd say that 90% of "AU" coins are no more than EF. An AU coin should have no signs of wear and be in all respects uncirculated but with perhaps a few bag marks or chatter over and above what you would normally expect. A few minor scratches where it was broken out of the String roll and dropped in the cashier's drawer. As the difference in cost from EF to AU can triple or more then it's important that the other party is also going to see it as AU too.
Also, the above rule of thumb should be applied with a dose of common sense. Modern, currently circulating coins are not usually worth more than face value unless in UNC or occasionally AU condition. A 5 year old coin in EF or less condition or less needs to have some scarcity to have any real value.
Non illegitimis carborundum est. Excellent advice for all coins.
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In some cases the difference is 3 times instead of 2. And for gold it could be 5%. Common silver can change in 5-10%. So just be reasonable and check similar coins. A good idea to check what large companies suggest in similar to your coin material.
I usually go with patterns to work out gaps. If a price drops from $120 in Unc, to $40 in XF to $13 in VF and I have a coin in F then the price is likely to be $4 in F because it has dropped by 2/3 each step.
Between AU and XF there isn't such a radical difference so I usually increase the XF value by 20%, this however depends on the percentage of value increase between XF and MS UNC
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It is definitely better then modern UK £2 UNC from mint packs.
XF for sure, AU or UNC - it is required to look with a magnifying glass. As SCWC suggested one needs 30x.
Quote: cyprusalexanderIt is definitely better then modern UK £2 UNC from mint packs.
XF for sure, AU or UNC - it is required to look with a magnifying glass. As SCWC suggested one needs 30x.
magnifying glass? I wouldn't even know what to look for. it has some scratches that I can notice without a glass, that I didn't notice at first.
Taking a break from swapping for a while, but still interested in pre 1799 Spanish coins, I will make time for that!
