The transfer agreement document was leaked last season (Football Leaks) the contract as a PDF file that showed it was £42m because of a huge amount we had to pay the third party owners, not Porto.
I posted links to show this on here not too long ago but was dismissed by some of our fans who are convinced this has to be about how we are hated by the football media, which is understandable given the track record (even with the way Sterling's transfer fee was Inflated in some periodicals) and whoever else hates us (Have they formed a queue yet?) but the evidence we have in front of us is hard to ignore if you are honest with yourseld.
It's all over the internet at this point.