You can ask the vet for an autopsy here I think. If you suspect he was sold for very cheap, Machi (dark green male, at 3 months) was sold to me from a different breeder for 18000 yen for reference, as that was cheaper than I thought as well for such an uncommon bird kept here in Japan.
In any case, I'm really sorry for your loss. I'm sure you did everything you could, and Apollo was as comfortable as he could be in his last moments.
Take your time to grieve, and I'd like to hear more about Chi and Fu, or about Apollo if it helps to talk about him, when you're feeling better!