I'd wager the data is pulled from news sources ie "Sky sources say an estimated £9m deal" or the like.
It's never ever going to be 100% of course but I'd say it's about as accurate as what Naylor reports in the Argus when we close a deal.
Yeah, TransferMarkt has it down as around £50m in transfers and loan fees which isn't much in the grand scheme of things. Never sure how much faith to put in the numbers on that site though so best use a hefty pinch of salt.