The price on faction vendors is a bit of a long story.
To answer your question, night shade is 2 gold, at its cheapest price. You will usually find many reagent faction vendors stacked together by a bank. This makes it very easy to buy quantities of night shade.
The long story part, is faction vendors are controlled by factions. That means that someone on your shard, in factions, has to place the vendors. It also means that for you to get the lowest price, the faction town finance minister has to set the lowest prices for that town. The price on vendors will vary alot depending on what faction folks are trying to accomplish.
The towns that can have faction Vendors are:
Brit, Skara, Moonglow, Minoc, Vesper, Yew, Trinsic and perhaps Magincia. I'm not sure about Magincia since there is an invasion going on. Faction Vendors can only be placed in town.
The good news is that most PvP folks consume alot of potions. That consumption means there are usually some vendors around. Take a walk around the Fel towns and see if you can find any. They might be hidden so only those that know about them can use them. If the vendors you find don't have the lowest prices, continue your search in the other towns.
I would also recommend stocking up on all the reagents for your new Alchemist. It's nice to have a stack of 60k of each reagent just in case you want to grind pots one night.
Happy Poisoning,
Q