One rule of thumb I used to follow was to take the price at a given grade then multiply by .65 or .70 to get the next lowest grade. So if a 7 is $1000, a 6 would be about $650-$700. This is only a guide and not a perfect formula.
This does not always hold true for certain cards or issues that see a huge jump from 7 to 8, or 8 to 9, etc.
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