Pricing armor parts is tricky, especialy atm considering the upcoming crafting changes.
First of all are you willing to buy some and trying to see if you're not getting ripped, or trying to determine prices for some you are selling?
IMO you can sort them in 3 categories to make it easy :
* medium resists and random mods : these are the ones with 50-55 total resists, and inadequate mods (ex : +3 int +1 hpr) ----> 0-1 mil depending on mods intensity.
* good resists and correct mods : while not being very nice, these are pieces with 60 or so total resists, and random mods of decent intensity. ----> 1-2 mils.
*very good resists and good mods : 65 + resists and useful mods (ones that go together well, could be lmc-mr, +dex-dci, etc...) ----> 3 mils and more.
Sometimes people craft exceptionaly good pieces, and those really have no bottom price. Depends a lot on who need it, and how badly they do.
Once again runic changes incoming, so it might not be the best time to invest massive amounts of money in armor parts.
Hope this can help.