Prices depend on your Charisma and the Hard Bargain perk

You can buy them for 20 caps, but sell prices are often a lot lower. You can increase the amount of caps you get for selling stuff as described in the comment above, but it will still be less than the buy value.

Don't put to much faith in what you read on the wiki. The Fallout is wiki full of opinion. Either because it's old information or information that wasn't really vetted. The wiki is also very hard to maintain because someone in their great wisdom decided to put all things Fallout into a single wiki (bad idea) when each game should realistically have its own wiki. I've spent hours upon hours updating various things in the wiki. It's a mess.

Also, don't expect to make more than 5 caps per purified water from a vendor. I wouldn't even bother trying to sell them to other players. There are better ways to make money fast.

Here's a sure fire way to make money fast in FO76:

1. Max out Hard Bargain

2. Sell lots of weapons and aid items to vendors. I can usually hit max daily vendor caps with 1.5-2 runs at Westek.

3. Collect a ton of easily farmable resources (wood, plastic, steel, etc). Convert it to bulk then sell that bulk to vendors. I usually get 8-10 caps per bulk item sold to a vendor.

Fandom, in their limitless wisdom and undeniable omniscience, does not allow so-called "duplicate" wikis. Making a wiki for an individual Fallout game would get shut down right away.