This proves that property is corrupt. Is there enough evidence?

No, this is suspicion, but it is not evidence. It is common that centralized purchasing is more expensive nowadays.

First of all, I agree with the depth of purchase, but this kind of collective purchase can't be compared with private purchase, such as installation of after-sales service, payment arrival time, and demand for more consumable parts ... I borrow a mind map with similar meaning:

You can't deny the depth of the purchase, but what you have alone can't be called evidence.