Strong Buy
Grading Gardevoir-EX (Steam Siege) currently looks like a clear win. A 870% Return on Grading comes from the spread between $108.48 raw and $1924.99 graded. Returns like this tend to attract sellers, so the raw price may not stay this low.
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)
This barely needs
a shot at a PSA 10 (0%) for this to pay off.
The PSA 9 price on its own almost covers what you'd spend, so you're paying very little for the chance at a 10 worth $1726.51.
Among the 34 cards in Steam Siege with PSA 10 sale data, Gardevoir-EX ranks 6th for Return on Grading and 3rd by PSA 10 price. Of the 2 Rare Secret cards with PSA 10 sales, it is 2nd for Return on Grading. The median PSA 10 in this set sells for $250.00. 22 of the 34 clear a 100% Return on Grading.