prices for unlisted/in-between grades?

If you have a value for the various grades for a particular coin, plot those values on a graph, the mark your coin on that graph, between the grades.

In reality, you only need to know the value for the grades immediately above and below your own, and your coin should sit between those grades.

After a while you get good at it. I do it by interpolation between the grades mentally, and how close to the known points your coin is. Knowing that the value graphs are exponential in nature helps.

DBM: You got it!
That is the mental system I use.
Gives the easiest approximation to the exponential curve.

I think DBM and sel ar correct...pricing tends to follow an exponential curve not a straight line as coingirl suggests....

Interesting thread! Here's my perspective: inbetween grades exist to fit those coins that are so darn close to the higher grade the differences are arguable but negligible. The inbetween grades exist to catch and coddle sliders. For instance, a VF-30 is far far closer to an EF-40 than it is to a VF-20. Pity that such associations have little effect on pricing. What I've found holds true is that usually an inbetween grade will garner 25% of the spread between two standard grades. I don't like it because I often sell but I have to accept it.