We saw that earlier Pablo Garcia-Cuervo reported a €25 million bid that Tottenham put in for Juan Mata, but at the time no one else had picked up on the bid. Garcia-Cuervo has a decent track record so there was reason to believe that Spurs did put a bid in, but now it looks like we have confirmation that Valencia do in fact have a bid in from Spurs. The Guardian now has the story too, with them saying it is a £22 million bid, or roughly €25 million.
So I think it's fair to move forward under the assumption that Tottenham really having put in a big bid for Mata. It outstrips any reported bid that Arsenal have put in for Mata, but the release clause in Mata's contract is just €21 million so if Arsenal can meet that then the decision is Mata's, not Valencia and the bigger transfer fee a Spurs transfer could net them.
How does Spurs convince Mata to choose them? It'll be tough considering that both Arsenal and Valencia offer him Champions League football, whereas Tottenham doesn't. Check that, it's impossible. There simply isn't a single reason for him to choose a move to Spurs.
There's also the question about why Spurs would want Mata. That's not to insult Mata because he is a fantastic player, but Tottenham have a gigantic hole at an expensive position, striker, while they have two good wingers in Gareth Bale and Aaron Lennon, while Rafael van der Vaart operates in the hole. They could sell Lennon, who did draw reported interest from Liverpool in the spring, or move Bale to left back, but neither is ideal. It's a good thing that Mata has no reason to pick Spurs over Arsenal or staying at Valencia then.