Im guessing you are doing ladies fashion shoes, not sport shoes?
In terms of your original question, i would say that PVC vs. PU would generally feel similar and indeed PVC is a bit cheaper. Also typically you can get more “fashion” finishes in PVC (ie. metallics, faux leathers, wild prints, etc.). Im not sure about the PVC importation rules.
As for costing, there are a lot of factors that make it a pretty complicated subject. A lot depends on the supplier you are working with #1. If you can trust them to provide honest quotations, and/or a complete CBD (cost break down), you will be further ahead then just guessing to start.
I would try to first get yourself a complete part by part cost break down. Each part on a CBD should show material cost (per sq.ft or 54" typically if synthetic), usage, loss, and then adjusted usage (incl. loss) and total cost per part. all details (ie printing, laces, etc.) should also be itemized.
You CBD should also show labor costs, fty overhead, thread+glue, packing ,etc. and fty/agent profit then your final FOB price. Once you have this (which many factories are unwilling to provide so they can hide true costs), you are better off to negociate and see which areas/parts are the ones that are adding up to cost you more than your target.
Looking at it this way (and working with the fty is they are willing to do so), you can focus on the “big ticket items” instead of spending time changing small parts that will only save you fractions fo a cent.
First things to look at would be expensive materials (like the nubuck you mention - maybe change to pig suede if you want to keep it leather, or there are also lots of nice synthetic nubucks (PU))., outsole costs, large embroidery or other expensive details, then labor costs, overhead and fty profit.
In general, though as first mentioned, a lot depends on your situation. If you are with an experienced fty, have a long relationship, and do millions of pairs per yr, its a different story that a new, low cost supplier with no forecast to offer…
Best of luck. A way to start might also be to approach you supplier and tell them your traget FOB price and ask for suggestions. Remember that you and the supplier are actually on the same side. You both have the same goal to make money and sell shoes and if they are overpriced they wont sell and then neither of you will be satisfied.