Q:

1960s-1990s third party lenses - why was the "lowest tier" cheaper to make? 

From a retrospective, it seems that there were a couple of mid-tier makers/importers like Vivitar, Sigma, Tamron, Soligor ... and that there was a plethora of small brands (for example, one brand starting with H and one with M...) that seem to have made consistently bad zooms and even some very sub-par primes.  Seems they also had a reputation for very mediocre quality back in the day.  Trying out these lenses, when they can be had for cheap, usually confirms yesterday's prejudice. 
The thing is, not every design by the mid-tier brands appears to rely on special technologies like ED/SD glasses or aspheres.  Neither was every design by these brands that DIDN'T use (or announce?)  such technologies mediocre. 
Also, the "lowest tier" zoom designs do not seem to be of consistently lower element count, while there have been good low-element prime designs then and before, making the explanation of "too few elements" moot. 
Additionally, most of the "lower tier" was NOT in the serious wide angle domain, where things like very precise centering, floating elements, or aforementioned special technologies are known to be essential.  And the flaws typical of these cheap lenses tend to be about contrast, color, or all-corners-equally-bad-on-all-samples distortion/spherical aberration/CA... , not about things that would be typical for a sloppily centered lens. 
Also, mechanical build quality tended to be decent with most ANY brand back then. 
So far, we can assume that anyone who wanted to build an 80-200mm variable f4-ish, or a 135mm f2.8, or a 35mm f2.8, or even a double gauss 50mm, had to buy a similar amount of similar kinds of glasses and have them ground and cemented into a similar number of elements and groups.  I am aware there were 3/4 element 135mm designs, but they tend to use at least one monstrously heavy chunk of glass, so hardly any material savings. 
So why was anyone able to save cost and sell cheaper by building a WORSE example of these old standards?  Was the price difference all about applied design knowledge and intellectual property?  Was it mostly about the coating science?  Were there methods that yielded relevantly flawed spherical lenses at lower cost?  

A:

This is easier to understand when we consider what made professional lenses during that time period more expensive.  While a dozen criteria (like optical design, lens coating, and weather sealing) contribute a little to the cost, production costs and quality thresholds have the most impact. 
Production costs
Consumer grade "lowest tier" lenses could be manufactured in batches in a highly automated fashion.  Hiring skilled technicians who produce professional "higher tier" lenses adds significant labor cost and is necessary to ensure each lens element is the highest grade.  Professional lenses would often be hand-assembled as well. 
Quality assurance
When a quality threshold is raised, the number of parts that fit the desired specification shrinks.  For example, Edmund Optics® is a manufacturer of spherical lenses and indicates they can produce 10x more commercial grade (low tier) lenses than they can produce high precision elements with the same surface material.  Essentially manufacture ends up re-purposing the lower quality lenses that don't make the cut (recycling the material or selling them at a lower cost are two options).