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The maths doesn't work on the Special Illustration Rare Sandy Shocks ex from Prismatic Evolutions right now. A 5% Return on Grading comes from the spread between $23.48 raw and $119.69 graded. Hold it raw and watch for the graded price to move.
Most grading maths asks you to guess your odds of a PSA 10 first. This works backwards from the prices instead and tells you what those odds would have to be.
The number is different on every card. It's how confident you'd need to be that this copy gems before grading pays for itself, so 20% means roughly a 1 in 5 shot and 50% means a coin flip.
Grade it if you rate your copy better than that, leave it raw if you don't.
It assumes a 9 is the worst you'll get. Centering and edge whitening send plenty of modern cards back at 8 or lower, and that's not priced in here, so treat this as the floor of the bar rather than the whole of it. Selling fees aren't counted either.
(cost - PSA 9) ÷ (PSA 10 - PSA 9)
You need at least
a copy you'd bet on gemming almost every time for this to pay off.
You'd be risking $88.48 to make $6.21, so the 10 has to be close to certain before this is worth doing.
Among the 105 cards in Prismatic Evolutions with PSA 10 sale data, Sandy Shocks ex ranks 34th for Return on Grading and 31st by PSA 10 price. Of the 32 Special Illustration Rare cards with PSA 10 sales, it is 19th for Return on Grading. The median PSA 10 in this set sells for $82.73. 14 of the 105 clear a 100% Return on Grading.
