When a low quality lab in China is producing something that normally costs such a high amount there are always corners cut, quality issues and people manufacturing it that are not using the same quality control and standards you would even get from a compounding pharmacy.
Author William Llewellyn's talk on how the black market of anabolic steroids evolved over the years explains how things like this are made and what sort of contaminates get into gray/black market injectables (regardless of oil or water based).
Anabolic Steroids: an evolving black market (28mins) https://youtu.be/0LL7bL4F9G4
The cost of GLP-1 drugs doesn't come from production costs or complexity, it is purely a function of being on patent.
A month's supply of Ozempic costs less than a dollar to produce. It costs $25k in the US because of patents.
That video is literally about anabolic steroids not peptides. Not to mention there was 0 testing labs in 2012 that users could send their items to, to get tested while there is many now a days. Back then in 2012 unless you had a friend at a University lab that knew analytical chemistry you were out of luck to get anything tested.
You aren't wrong about a lab in China cutting corners, having quality issues and low quality control standards but people don't care if they see 10,000+ people using peptides for decades without a single complaint about infection or real problems.
"is producing something that normally costs such a high amount" the cost has very little to do with the chemicals themself, the cost is about recouping R&D costs and having a 90% profit margin.
I see this error on HN a lot, but the overwhelming majority of people will not do that, because the overwhelming majority of people take the path of least resistance and neither have the domain knowledge nor the drive to figure out how to do things outside the easily packaged path. Your example class of peptide-using bodybuilders are probably in the top 1% of both drive and domain knowledge to have gotten there (probably higher, even - less than 1 in 100 people on the street are that jacked).
A lot of HN users are extremely high-agency people with a significantly-above-average ability to understand how systems work and how to take advantage of that. A lot of said users make the understandable mistake that most people are like them. This is not the case.
It is extremely important if you are in charge of designing a system (as many of us are) that you understand and internalize that any claim of "people will just" that requires more than a few steps will only bear out for a very small minority (which are important edge cases, but should not be your expectation of the average user)
There is Facebook groups with thousands of 50 year old moms taking gray market peptides, not just top 1% peptide-using bodybuilders. My example of peptide using bodybuilders was about the anecdotal safety of peptides. The barrier to entry previously was ALOT higher, you practically had to either contact a middleman in China and wire money to a random bank account overseas or buy it from a steroid drug dealer, now a days there is thousands of online storefronts that accept credit card and ship domestically that come with visual graphic instructions on how to do everything.
I'm sure when the New York Times writes a big piece about how gray market peptides have become a huge thing in the Fitness social media space, others will recognize it is growing and growing fast.
A 2 year supply of Ozempic (Semaglutide) lyophilized in sterile vials is ~$120USD (300mg of Semaglutide, 2.4mg a week at max dose so 125 weeks) on the black/gray market and that's with at least 2 middlemen making a profit so realistically the cost might be closer to $70USD?. Anyone can pay to get it HPLC tested to confirm the quantity, purity, sterility etc not to mention people (bodybuilders) have been using gray market peptides for 10+years and you never hear any stories about something going wrong (Things go wrong with oil based steroids for bodybuilders commonly not peptides in BAC water).
Meanwhile that same ~$120USD 2 year supply of Ozempic is $8400 in Europe, $9600 in Canada and $24000 in the USA.