Yes, that's true, and except in some cases like Pogba's transfer we don't know the exact figures for anything. However, wage reporting is especially speculative. For example when Pogba joined us he was speculated to be on 250-300k, until some of the documents 'leaked' due to that FIFA investigation suggested it was closer to 160k. Or how Ashley Young's salary is skewed by how he chose to have his signing-on fee distributed across his contract. Transfer fees once the transfer is done and dusted generally tend to be reported in the right ballpark at least.Even the fees paid are undisclosed but hey lets just ignore that. Every transfer rumour is speculation. Every thing is.
We also usually learn nothing about things like signing-on fee, agent fees, appearance fees etc. Trying to examine a transfer in such financial detail is just beyond our knowledge as fans.