I would guess that it’s less liquid than the norm for Great Houses, given that the North is less developed as an economy, and that in-kind taxation is way more important in the winter in the North than it is in other regions less affected by the climate.
So I would imagine that a good part of their wealth probably went up in smoke when Ramsay sacked the castle because he’s that stupid, but some remains (I would be very surprised if there aren’t hidden storehouses in the crypts) and I would imagine a lot is being held in trust by their bannermen.