I mean for anyone who reads subtext well it was pretty clear the merchant was taking our boy for a ride. Two talents the first time, that's close enough to what i'd imagine market price is to not be noteworthy. After all Ming Hui shelled out a lot for the eggs which were silver bullets for his cooking challenge, and several spirit beast parts were 2 talents each when we when hunting with Nokai. So uncut stones of uncertain Color or Clarity, without much anam on their own from a nearby mine that the sect has? Kinda high, not unbearably so, but still.
Then the next batch is 3 talents, that's a 50% mark up with no real drop is supply and likely an only small increase in demand as i assume even with all his fighting bihai took some time to go through his first batch. If something hit the sect's mines that hard for what would be junk stones in most cases, you can bet we'd hear about it. Doubly so since Diamonds are not all that valuable to cultivators, typically the thigns they want are filled with anam and this... isn't. It's still money to the mortal jewelers and mortal wealthy, but to a cultivator it's just a pretty stone normally. Then he jumps the price to FIVE talents, which is a huge mark up from even 3 without so much as a "I am sorry honored cultivator" which most sensible people would do in the face of an angry teenage demigod who could hand them their own ass as a matter of course.
The merchant was taking Bihai for a ride, not trying to keep repeat business, not trying to mend fences orvoid a confrontation. The entire conversation reads to me like the Merchant is using the poorer, less economically inclined disciples, aka the new kids, as people he can fleece quickly with shinies. He tried the same approach with Bihai, so he lost 1.5 talents (assuming he was telligng the truth on the estimate) of diamonds. Which cost him the talents he'd have gotten extra if he scammed Bihai successfully.
Now Bihai doing this to a person dealing in good faith who had a bad thing happen and now need to rise prices, that'd be evil. This, in my opinion while not entirely warranted why giving a jerkass merchant scam artist his just deserts.