In your example this means idealists think the worth of a coin comes from itself, while materialists ask for the social processes that this property "worth" emerges from. This would have to be analysed both economically and in termns of constructivism

Idealists imagine ideas to have their own realm of existence with a mystical source of power, indepentend from said dialectic.

No we don't. We believe the supposed world of matter is a social construct created by people's belief, and that dialectics occur within the mind. We believe the "material" world arises as a result of mental dialectics. Meanwhile, materialists believe that matter just popped into existence in its own for no reason, with no cause, and that all we experience is matter.

For example, take a trans woman who has not yet begun HRT. According to the materialist, her body's male features are the true nature of reality, and our perceptions arise directly from her male body. The materialist refuses to bear any responsibility for perceiving her as male. Meanwhile, an idealist says that maleness is a social construct, and the true nature of this trans woman is her female identity. Her body is a visual symbol created by our minds and existing only within our minds. We bear responsibility for how we create this symbol. Perceiving her body as male is in most situations an act of violence. We have the choice to perceive her as female, and we should do so if that is her wish.

The way we perceive her body is informed by thousands of years of history of society, a dialectical process of causation intertwined with he patriarchy and the ideals of the enlightenment. A materialist denies all of this complexity and says that their perception of her body as male is objective truth, which simply appeared on its own with no social process informing its creation. They maintain the body is pure physics and their mind has no impact. This is irresponsible and dangerous.