The rule in Spain is that the player has to deposit the buy-out clause himself.
That means that clubs somehow have to get the money to the player who wants to leave and every way of doing this involves income tax, which amounts to around 50% in Spain (and most countries in Europe for that matter).
So logically, the actual amount the clubs would have to pay would be about twice the buy-out clause, however, I've seen various newspapers mention a figure that is 75% higher than the buy-out clause, which would be 63m in Laporte's case.
Lately, clubs have found a way to somehow bypass this (e.g. Bayern who supposedly didn't pay any kind of tax for Martinez) but I'm not sure how they do it or whether it's completely legal. Maybe they just lie about it and don't disclose the actual figure which seems to be en vogue nowadays.
That said, I don't see Zubi coming up with such a sneaky attempt at buying Laporte because he has a long history with Bilbao and is still very fond of the club, judging from comments he's constantly making.
So for now, the only reference we have is newspapers and Spanish law and both suggest that Laporte's eventual fee wouldn't be below 60m.