Please keep in mind that my responses are extremely simplistic and ignores a boat-load of other relevant factors...
The quality of the plastic and who makes it
does come into play. That is where the abbe values, lens design etc come in. Then you also get
lenses and lenses. You might e.g. cope with cheap "injection molded" (there is a better term but it has slipped my mind) lenses that some local labs are playing with, due to factors like a low RX, while I may not.
Given the same script (RX) an entry level cheap pair of multifocal lenses will never give you as much range and reading ability as a top of the line Zeiss or Hoya backed multifocal lens. Zeiss likes to brag as to how they are using the masses of prescriptions they have processed over the years to optimize their premium products to give the wearer a much more comfortable experience. Even Hoya uses their software (and pay them click-fees). Improvements in technology has removed a lot of the previous factors that gave multifocal lenses a bad name. As new products are introduced (at a cost-premium), older (cheaper) products are phased out.
You get what you pay for in the end.
At what price should spectacles be sold in your opinion?