$900 tuners exist because $600 tuners used to be the most expensive tuners you could buy, so someone decided he wanted something even more exclusive. If tomorrow, someone makes a $1200 set of tuners, they will sell to the very same person who was once satisfied with his $900 tuners.
It's really no different than why a man pays $150,000 for a Patek Phillipe.
It's because he wants to set himself apart from the man who paid $25,000 for a Audemars Piguet, who was setting himself apart from the man who paid $9000 for a Rolex, who was setting himself apart from the man who paid $80 for a Seiko, who was setting himself apart from the man who kept time with his cel phone.
This "one-upsmanship" Y Factor is essentially 100% of what drives the high end of any market. Doesn't matter if it's guitars, guitar tuners, wristwatches, the man who insists that only Lie Nielsen tools will do for his workshop, the man who spends $300,000 to buy a 1959 Les Paul, or $3,000,000 for a Stradivarius.
Mans desire to set himself apart from the pack with devices that conspicuously imply his 'superiority' has its basis in hard-wired impulses of human ego. The phenomena detaches itself from rational thought and reason pretty early on during it's evolution.