Chelsea are reportedly hoping to sew up a move for Isco quickly in order to avoid a bidding war with Premier League rivals Arsenal and Manchester City.
The Spaniard has fallen out of favour at Real Madrid under Santiago Solari and could be poised to leave the club in January if the right offer comes in.
But there are unlikely to be any takers at that price and a £70m deal would still represent a profit of over £40m on the fee Real paid Malaga for Isco in 2013.
The deal would also include a hefty signing-on fee and Isco would sign a four-and-a-half year contract at Stamford Bridge, keeping him at the club until the summer of 2023.
Tuesday's reports claimed that Manchester City were in pole position to sign Isco, but other journalists pointed out that City are more interested in signing a central midfielder and a left back before filling any attacking positions, where they are well-stocked.
Arsenal have also been mentioned in passing, but there is no evidence that the Gunners are in the process of making a firm bid for Isco.
Isco is yet to start a La Liga match under Solari amid accusations that he is overweight. Florentino Perez has agreed to sanction the sale of Isco if his asking price is met.