A suggestion: Average the catalogue price between the lower and higher grades. If the note is VF30, it's the average between VF20 and EF40 prices; If it's a grade other than the straight average — say VF25 rather than VF30 — calculate the average between the VF30 price and the VF20 price. if it's a F15, calculate a third more than the standard Fine 12 price; for an F18, add two thirds or thereabouts from F12. Same for Unc grades between say 60 and 63 if the note is graded 62 or the ghastly TPG grade of 61. The grade of Unc. 63 is the "regular" choice uncirculated grade. If it's a 64, average the 63 and 65 prices. Same for all other grades from top to bottom.