I think what happened with the Baines to Man U stories is a person inside Man U was leaking this to his source than Man U were interested and what they hoped to get him for. The reason it kept coming up was the same person in the Man U press office was leaking it, as part of a strategy in the hope Baines would force through a move himself, as we all know from what Baines has since said this didn't happen.
It's my understanding, the way the general shot in the dark rumours work, is there are freelance journalists who have sources within certain clubs and they get proper leaks of who people are interested in. Example, the rumours we're looking at the likes of Rodwell, Cleverly, Welbeck etc, I have no doubt that our scouts have been looking at them as part of a long list of options, whichever freelance reporters have connections with our club will get these names, they're known within the tabloid industry to have genuine connections to the club, so the tabloid will buy the story from them and it fills pages. 90% of the time, these rumours don't materialise because managers look at the entire list, think of costs and then choose to pursue the ones they want. Doesn't mean the others weren't under consideration, but the options we did go after are the ones we wanted most.
I think the fee and the player exchange on the other hand are guess work or throw away comments by the source at best. But I have no doubt Barkley is probably on City's radar, along with a whole host of other options. But by the fact no other tabloid has run with it as a backpage story and the amount quoted is nowhere near what most sane people would expect him to move for, I don't think this story will be much of a goer this window.
I think that's what sets these tabloid rumours apart from the Twitter ITKs however, who literally are just logical guess work based on the stature of a club, the position they're in need and the players available. I do think most tabloid rumours always come from someone with some sort of connection within a club involved, it's just most of the time, they're massively exaggerated from being on a consideration/scouting list to being imminent bids. If a bid really was imminent then it'd be all over all the papers as the top journos would have all had briefings from the club looking to unsettle the player.