For completing some quests or obtaining a certain reputation with some merchants/vendors, you can get discounts on their wares. Problem is, with just a Good-Natured Rascal reputation, you can get 95% discount from Dr. Usanagi. There are several other merchants that have very high discounts. You can't get better deal than 1:1, but it still gets you super good deals at super low barter. This ruins harsh economy mods that make buy/sell prices a lot tougher.  Below is a list of merchants with discounts that I know of in their vanilla state. If you found some I missed, let me know. Price hikes are not halved.

YUP fixes are forwarded and YUP is required.

Chet: 50/75 -> 18/37 (75% discount if Mixed reputation, 50% if not + finished Run Goodsprings Run)
Trudy:75 -> 37
Mr. Holdout:                             40/80/85 -> 14/28/42
Cliff Briscoe:    25/30/35 -> 5/11/17
Jack:65/70/75/80 -> 10/20/30/40
Usanagi:  95 -> 47
Mick/Ralph:    10/15/20/25/30 -> 3/6/9/12/15
Torres:             15/20/25/30/35 -> 3/7/10/14/17
Street Vendor:        40/80 -> 20/40 (In the strip)
Dr. Ada Strauss:     10/15/20/25/30 -> 3/6/9/12/15
Great Khan Armorer:      75/80/85 -> 14/28/42
Gourmand Food Supplier:     50/75 -> 18/37
Kemp:     10/20 -> 5/10
Farkas:    20/30/40/50/60 -> 10/15/20/25/30
Farber:
Farber is unique because what is done for him and how affects discounts.
Nash: Trade for caps (+2) or for food/water (+5) -> (1/3)
Fitz:  Trade for caps (+3) or for food/water (+5) -> (2/3)
Crimson Caravan:  Trade for caps (+5) or for food/water (+8) -> (2/4)
Kitchen Machine: +5 -> 3
Vault 22 Additive: +3 -> 2

Usually command to trade is showbartermenu. Adding a number after it denotes the discount/extra markup. Showbartermenu 95 = 95% discount. Negative values are extra markup. I spaced out discounts evenly based on reputation (25/30/35/40% -> 10/20/30/40% discount based on reputation). You can  change it yourself if you disagree with my opinions,  but make sure to save with GECK and not xedit.

Farber, Kemp, and Farkas have special commands for giving discounts, because rather than doing "showbartermenu number" the number is a variable found elsewhere. Farkas has her calculation in onbegin part of the dialogue, which is a multipler (originally 10, now 5 to halve it) multiplied by reputation. Kemp's setting is found on Morales' ending dialogue for the quest. Farber's is set between dialogues, but it may be easier to compare different quest options by going to VFSCampMcCarranQuest.