Jorge,
While synthetic stocks have been around for close to century (Bakelite was an early material), the development of what are known as "lay-up" stocks, made of various synthetic fibers bonded with epoxy, took place in benchrest shooting in the 1960s. This was due to the desire for more stable AND lighter stocks, so more weight could go into barrels, scopes, etc., resulting in better accuracy
The lay-up developments eventually moved into hunting, due to the desires of many mountain hunters for lighter weight, plus stability and strength--and the ability to use "standard" length and weight barrels, instead of the short, skinny barrels often used on lightweight "classic" rifles. This resulted in lighter rifles that balanced better, and shot very accurately.
Which is why some major manufacturers started putting lay-ups on factory rifles, as I recall in the late 70s and early 80s. But they did not cost less than walnut-stocked rifles. Instead they cost more, because lay-ups require more time in manufacturing, and some of the materials, such as Kevlar, can be relatively costly as well.
The reason a lot of synthetic-stocked factory rifles cost less and often weigh close to the same as walnut-stocked rifles is faster, cheaper methods of making synthetic stocks, particularly injection-molding--which in a way is similar to the increasing use of hammer-forged barrels by American companies. The initial tooling for both injection-molded stocks and hammer-forged barrels is very expensive, but after that both stocks and barrels can be cranked out quicker and cheaper than more traditional methods.
One early maker of injection-molded stocks was Bill Heckerman of Belgrade, Montana, who developed Butler Creek stocks for various factory rifles in the 1980s. He told me back then that the initial molds cost around $250,000, but after that stocks could be cranked out for about $7 apiece. However, they did NOT weigh any less than the same basic shape of stock made out of plain-grade walnut, so weren't intended to reduce weight of the overall rifles. Same deal with Ramline stocks, another early IM stock. Instead both Butler Creek and Ramline stocks were intended as cheaper, more rugged alternatives to cheap walnut stocks--partly because the price of walnut was not going down.
Both hammer-forging and injection-molding are why companies like Ruger can now make extremely accurate rifles that sell for relatively little money. But they were never intended to be significantly lighter in weight, as lighter synthetic stocks still cost more.
This is one reason custom lightweight synthetic-stocked rifles tend to cost considerably more than the fewer "classic" factory rifles being made today. So a lot depends on what kind of rifle we're talking about, whether synthetic- or wood-stocked.
One more recent trend in "classic" hunting rifles is limited production models, because demand simply isn't enough to make them regular items--but some hunters will pay somewhat more for them, especially if they're chambered in "classic" cartridges. I'm kind of a sucker for these myself, but almost by definition they're not selling like everyday synthetic-stocked rifles.
