Update: Marca (yeah, I know) are reporting that the transfer fee is a total of €28M with Mata offered a five-year deal with Chelsea. Wages unknown, and this is still unconfirmed.
Well well well. We knew that the Juan Mata to Chelsea rumours were heating up, but I don't think anyone expected anything to happen this fast: Chelsea have apparently made a bid for the Valencia winger, and the club has accepted, with Mata on the way to Stamford Bridge. Rumour is flying all over the twitterverse, but it seems mostly to be sourced from Spanish sportscaster Juanma Castano. I don't know much about Castano, but there doesn't seem to be much reason to doubt him.
Mata se va al Chelsea. Información. No opinión. Noticia de ahora mismo.less than a minute ago via Twitter for iPhone 
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Juanma Castaño
juanmacastano
(Full disclosure: That's two tweets condensed into one)
That translates to, 'Mata goes to Chelsea. [This is] information, not opinion. Breaking news.' Pretty unambiguous, I'd say, although I'd be a little worried about taking anything as gospel until more outlets are picking it up (not that foreign media sources are bad, it's just that I'm bad at picking up nuance). It's also unclear how far in the transfer process the clubs are, with some sources saying that Mata is yet to agree to leave Valencia and some seemingly far more sure that this is actually going to happen.
If this is legit (and again, there's no reason to believe it isn't apart from healthy skepticism of anything and everything said during transfer window time), we could see Mata at Stamford Bridge as early as next week. The last rumoured offer was around £27M, which is about 5M more (or one Oriol Romeu, if we want to count in more reasonable measures) than what we could apparently have had him four in July. That's not fantastic value, but it's still a very good buy - this addresses one of Chelsea's major needs with a very good 23-year-old player who'd have some resale value if he doesn't work out at the club.
I like.