india has a very weird medical economy at times, geared to the costs necessary to deliver ultra-basic care to many people who are dirt-poor, utilizing generics (occasionally of questionable quality), the ultra-cheap cost of labor, etc.
for example centchroman is an interesting case-study... the government basically paid to develop and take it through trials and it was generic from day-1, and then the generics manufacturers go to work and you have birth control that can be delivered to market for under a dollar a month (4 pills) in retail quantities, and the government distributes them free because it's cheaper than pregnancy/social services.
https://en.wikipedia.org/wiki/Ormeloxifene
Not that at the high level, the care isn't great, but, a lot of india is super poor and underdeveloped and delivering core medicines cheaply is relatively effective to provide a super basic level of care, they have a focus on delivering super cheap generics that cover the basic use-cases. I'm sure the glasses are cheap shitty molded plastic or something, or at most a super basic molded glass lens, perhaps with some subsidies. I'm mentally imagining the "BCG" glasses from basic training, designed to be indestructible and unscratchable (aka the birth control glasses).
https://en.wikipedia.org/wiki/GI_glasses
But that is the 80% solution, make em cheap and polish em up with super cheap labor and with basic materials you probably can get that down to 2 bucks or within subsidy distance of that.
Sure, it's enough to pay for the materials (frames cost pennies, and cheap lenses maybe $1?).
But someone still has to cut the the lenses to size, and fit them to the frame. This requires both equipment and skill.
I don't think I've ever paid less than $15-$20 for prescription single-vision glasses. I've paid more even in the 'wholesale' optical market in Beijing.
$2 is easily achievable for reading glasses, which are mass produced with lenses installed at the factory.