$70 for a 6S screen is cheap. Too cheap. So if you're aiming for $40 it means you're aiming for trouble. Unless you meant 5s?
Screens are made of LCDs, digitizers, flat cables, and a frame. And a few other components.
The type of parts used, and how they are assembled and finished all contribute to how stable the screen is going to be.
Just to name a few: bad screens will often lift from the frame, break all too easily, hang all too often, start with or develop yellowish spots, have bleeding LCDs, etc.
I find it more economical to install a good quality screen assembly, charge a premium, and warranty it knowing it will likely not come back, than install a bad quality part, charge cheap and make less money, not warranty it (or do but cost you a lot of money) and end up with bad reviews and a bad reputation.
The german have a nice saying. They often say they're not rich enough to afford cheap products. If you're starting up, you should basically think like a german.
Thus learn to audit the screens you are purchasing, and request the best possible quality. iFixit have some of the best parts out there, but you need a "Pro" account to get a professional's discount. And they stand behind their parts with a lifetime warranty on most parts I think.
There are other good quality / honest sellers out there. But often finding and keeping a cheaper supplier end up costing more because of the poor quality parts you'll have to eat until you find someone good. And then that suppliers may flip on you not because they are not honest, but because they also have little control themselves. iFixit are known to stand behind their product in case such an issue occurs.
Good luck with your hunt, or good luck finding a new business endeavour if you decide to stick with cheap parts.