Is this correct?

+ =

and

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or do the first four "volumes" make up "part 1 " ?

I don't know the answer to your question, but the first box set has 25 episodes and the first two individual volumes claim to have 5 eps each. So I'm assuming no.

I think part 1 might be the first four volumes. I don't see why someone would pay $120 for part 1 when the first two volumes are only like $30. But then again I don't see why someone would pay $120 for part 1 when the first 4 volumes are $60.

That still can't be right, though. All four of those volumes still don't add up to the 25 eps in the Part 1 box set.
And one reason to pay so much for the box is because the Part 1 set was released LONG before the individual volumes all saw the light of day. I still need to get the Part 2 box...

Today is the last day that Part 2 is on sale at Deep Discount for $48. That's why I am trying to figure out if I can get the first part cheaper than $120.

Part 1 contains Vol.1 to Vol.5. It also includes a bonus extras disc and a T-shirt. Vol.1 to Vol.4 were released via singles. Vol.5 was pre-solicited, but it was canceled by SPHE.

Part 2 contains Vol.6 to Vol.10. It also includes a beanie skullcap. The singles were never released separately. The release of Part 2 was delayed due to the theatrical and DVD/BD releases of the live-action movie feature.