Remember that growth is relative. GDP per capita in the mid 30s was still three times higher in countries like the states, UK, and Switzerland compared to the Soviet Union. This trend continued into the next decades. Pretty much all of Europe had a stronger economy. And there weren’t mass famines and rampant scarcity issues to the same extent in the west. Yes the Soviet economy did grow, but the libertarian argument is about efficiency.
And sure, price in isolation isn’t a super useful indicator. But many factors influence price (competition from profit seeking, availability of resources, etc). As for the latter part, companies do run market research, including non hostile espionage, to find what consumers want most. I personally don’t see where that would be irrational. It directly fills needs.