PCB vias impact in cost
Vendors are not good at telling you how costs will vary based on different specifications. They will always ask you for your files and provide a quote to that specific board. They generally can't, or won't, give you a relative answer. In my experience the number of vias is not as important as the size or type. Microvias (definitions may vary from vendor to vendor) are much smaller and will cost more than a standard via. Blind vias (which do not extend through the entire board) will also be more expensive. The price difference between having 200 vias and 100 is probably going to be negligible. Back when holes were drilled manually the cost would have been impacted much more, but nowadays with machines doing all the work, it really doesn't make much of a difference. Keep your via diameters large whenever possible, though. If there's a higher chance of a bit breaking (because it's so small), the board house will charge you more for the fab.