Very few price guides include in between grades like VG10 or F15. Is there a way to extrapolate values for those (I know that it's a bad idea to overly rely on guides but I'm still curious)? For example, if VG08 is $80 and F12 is $100, would it be safe to say a VG10 would be $90? I know that it probably is a coin-to-coin basis sort of thing but I'd still like to know other's opinions.
Well no, since you already know that the prices in most price lists are unreliable, extrapolating them wouldn't do you much good. You would do much better looking at comparable completed auction listings to approximate a good retail price.
Let's assume the price guide WAS reliable. Even then its not really easy to extrapolate. In most instances you could, but there would still be times you couldn't. Lets say there are a ton of collectors trying to put together a VF set of a series, and VF is $150 and F is $80. How much would a F+ be worth? You might guess $115, but if the demand is at VF, and not below, then the real answer is $80. In that particular case, unless it was fully VF, it was not worth a cent over F prices. There are so many instances of this kind of thing, that even IF price guides were very reliable, it STILL would not be possible to guesstimate a tweener grade.
There are no hard and fast rules, as price is driven by supply and demand, but I've developed an approximation that works very well for me. If the rarity of the coin is roughly the same in both the higher and lower grades (I don't advocate this method for 1927-S SLQ between VF20 & XF40, for example), and the intermediate grade is midway between the other two (applies for VF25 between VF20 & VF30, but not for VF25 between VF20 & XF40), I multiply the price of the lower by the square root of the ratio of the higher price to the lower price. Clear as mud? . . . Absent a calculator, I do the above as well as my noggin' permits.
If you have an intermediate, I would assume the value is the stated of the grade below until proven otherwise. If 12 is 10 dollars and a 16 is 20 dollars, assume your 15 is 10 dollars as well until you see what the market will bear.
The in-between grades aren't usually listed because they are usually not as common. Also, no offense to Sheldon, the intermediate grades and prices are not linear. A 20 is not usually 2x the price of a 10 - usually it is more than 2x. Therefore, you can't just linearly interpret the price. I often buy EF-45 Bust halves. The formula I usually use is: take the difference between the 40 and 50 prices, and divide by 3. Add that to the 40 price, and that's usually what I'm hoping to pay for a 45. It's not foolproof, and obviously price guides are more of a suggestion (so many things will affect value), but that gives me a rough inkling of an idea.
