I initially thought this link was ridiculous as he would cost way too much, but it turns out Valencia had a clause to sign him for 17 million euros at the end of his loan, which they can't now afford, so it seems he is available for a fee that isn't over the top.
If that article above is true then the 15 million euros bid makes some sense, as we like to try and pay a bit less by paying everything upfront.
...whilst on his arse