I have a feeling those tools don't cost so much for the nobles to make. They are just selling them for a really high profit for two reasons: 1) Easy money from wealthy commoners like merchants, 2) Most mana possessing commoners will simply die off and won't thus form a threat for the existing social structure (the nobles' power).
However, I really hope this series does have a good explanation for why there's apparently no such natural way to use mana that people wouldn't simply discover it on their own. After all, it's impossible for it rely on sophisticated tools alone because that would mean all people with such a potential would have died before the tools were developed. One possibility, among many, would be that all the magic genes come from an area where some substance that allows channeling mana out of the body is plentiful in nature (special plants, crystals, whatever) and it's useful in some way (like being able to fight with it). When people moved away from there, to places where that mysterious matter doesn't exist or is rare, people started dying. Obviously some would have known about what it's all about, but they kept the information for themselves, importing the substance X, and used their magical power to rule the plebs. Centuries later it would have become very restricted knowledge anywhere but in the original place. As we have learned, most people never travel anywhere and books are very rare, so information simply doesn't spread.