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Sending Yamper #99 from Phantasmal Flames to PSA would most likely lose money today. Raw copies trade around $2.93 while PSA 10s sell for about $53.99, a -42% Return on Grading. Grading and shipping fees would eat whatever margin is there.
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)
Not worth grading at any odds
Even a guaranteed PSA 10 ($53.99) wouldn't cover your $92.93 cost (raw + grading + shipping).
A clean raw copy is worth more to you than a slabbed one here. Sell it as is, or keep it for the collection.
Among the 50 cards in Phantasmal Flames with PSA 10 sale data, Yamper ranks 13th for Return on Grading and 16th by PSA 10 price. Of the 13 Illustration Rare cards with PSA 10 sales, it is 6th for Return on Grading. The median PSA 10 in this set sells for $40.48. 2 of the 50 clear a 100% Return on Grading.
