1) He's 28 years old
2) He wants ridiculous wages
3) Milan are reluctant to sell
Meanwhile, Martinez is 23 and is (reportedly) willing to lower his wages for us.
If Thiago Silva is 28 y.o., then Javi is 24 and Athletic are even more reluctant to sell then Milan because not only that they don't need the money, but they won't be able to replace JM with someone even remotely close to his value.
And I would not be so sure that he would be willing to lower his wages only to come to Barca. From what I read in the spanish press, not only that the VAT increased from yesterday, but if Javi comes to us he will go from paying taxes of 23% to 56%(Catalunya is the region with the highest taxes from Spain). Also if Barca wants to pay his realease clause they will have to pay all the money at once and at this moment the club only has half of those money.
And if 50million euros for Neymar are too much money, then the same price for someone who is coming as a back-up is outrageous.