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Wildcall
So how good is Manifest? I think Manifest is somewhere between "Better than a vanilla 2/2 creature like the ones generated by Huntmaster of the Fells" and "Worth spending a card to do once."
The X +1/+1 counters are a neat trick, but this is a bad Gelatinous Genesis for the mana you're investing. If you flip a Hooded Hydra, hooray, you're a hero. I just think Manifest is good when you do it every turn for free, not because you spent a card to do it once. If Manifest is so good you need this card, I could see it being a few bucks, but I think most sets have bulk rares better than this.
I think you are underrating this. This is a 3/3 for 3 mana with an upside. Compare that to a 1/1 for 3 mana on the ooze card you listed. For 5 mana this is a 5/5, compared to two 2/2’s with the ooze.
Once you get silly high the ooze is great, but being so much better at low values, plus the flexibility and the potential upside of flipping a manifested card…This thing may not be money but it is a great card.