OK, there are about 4000 marbles per basketball, by volume.
I can only guess how much grief we pulled out of Hildegard, but between a 20m set of wings and hundreds of basketball spheres, I'm guessing probably in the neighborhood of 400-500 basketballs of grief, total. (Wing estimate is 20m x 2m x 2cm)
Overall, what we pulled out of Hildegard is probably in the range of 1.5 to 2 million marbles.
If the Incubators were capable of pulling out that much grief from a grief seed, the 400 marbles we had after the Oriko fight should have been completely inconsequential. Likewise, a single grief marble should have been a laughable amount of grief when we handed that first one to Kyuubey, but he treated it as 'minor' — nice try, but a grief seed would be much better.
Digging through trying to find a cleanse-to-marble conversion, I got this:
So a cleansing of 20% of a soul gem comes out to 7 marbles. A 99% cleansing would thus be about 35 marbles. Edit: We also know that a grief seed is capable of holding at least enough grief for a 99% cleansing, from timeline 3 in the anime.
If a typical serious cleansing was done each time you got a bit above 50%, that would be about 20 marbles per cleanse. How many cleanses per grief seed? Mami's usage is very sparse, with one seed every couple weeks. And it was several hours of timestop to get Homura up to 20% full. I would not be very surprised to find that it takes maybe 40 marbles' worth to take a grief seed from 'unused' to 'full'.
We really should be able to answer that, but I don't think the specifics have ever been given.
In any case, if the Incubators can only draw on the surface grief from a grief seed — the grief added by using them for cleansing — that would mean each grief seed is worth about 40 marbles of grief, and pulling out 400+ marbles is a good 10 grief seeds' worth. That's the only way to explain that amount being surprising to Kyuubey.
We're still left at a bit of a value imbalance once we pull the 'real' grief out of a seed, though.
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