I think a half grade value would usually be the average of the grade below and above it. If a PSA 4 was $50 and a PSA 5 was $100, I would expect a PSA 4.5 to be around $75 if the card really looks like a 4.5 grade. The only time I believe this would be different woud be for higher grades like a 9.5 when maybe a couple 10's exist but only for pre 1980 cards, or if the half grade was the highest graded. As far as percentage increases, it would be different for each type of card and also different bewteen a common and hof card.
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