I bought a couple stripped lowers at a gun show. They had a whole bunch of them strung on a steel cable through the magwells. They were many different brands.
There were differences in finish.
Some were probably teflon-coated, whereas some were hard anodized. Some were semi-gloss black, some were rougher flat black.
Some of the machining around the trigger guard was slightly different, 1/16" here or there, but nothing functionally different. All had rounded edges where you'd want them.
On the inside, they were identical. If one would need a lot of machining to fit an M16 sear, so would any of the others -- not that I would care, since I'm not in any of the priveleged groups who might be able to get a new full-auto firearm anyway.
Now there are some lowers out there, made of various materials. But they vary in price. A $125 lower from a reputable manufacturer is not going to be made differently from a $125 from another.
Furthermore, Cavalry Arms makes one-piece molded lower halves and they run well. They have a lifetime warranty, too.
The lower is not the barrel. It doesn't have the same sorts of stresses on it. It needs to be made well, but $125 should cover that.