Cost difference of different Vias
It isn't a per-via cost. A normal PCB has all of its layers etched, laminated together, drilled and then plated. If you have blind or buried vias, regardless of how many there are, then multiple passes of the last three steps are required, and this is where the extra cost comes from. In other words, the cost of the whole board goes up as soon as you make the decision to use blind and/or buried vias.