If supply and demand are in equilibrium the value of a commodity is equal to the socially necessary labor-power needed to produce said commodity. Supply and demand skew this value in one direction or the other: higher market price (for example, gold) or lower market price.
My personal belief is that Marxist economics is more of a reaction to what was in his day a perhaps far less regulated and seemingly more corrupt capitalistic system (i.e. not enough corporate laws, not enough government intervention, not enough socialist schemes, not enough charities and the abuse of power by royal families and their imperialism).
Today, while capitalism is far more aggressive it is (I'd like to think it is at least) regulated a bit better. We've had decades, centuries to practice this system. Capitalism is perhaps the world's most successful economic system and coupled with sensibility and more liberal schools of political thought capitalism is perhaps here to stay for a long time.
Supply and demand is what drives our economics. For example if a labourer spends fifty hours producing a product, he'll charge people based on that work. But if demand for that increases, he will have to increase his own labour input or expand the labour force required to produce the product in less time and in greater quantity. I understand the supply/demand equilibrium is good enough, although this does fluctuate and if demand falls below a certain margin and profits can no longer be made, well then obviously the product goes out of production.
Now in a Communist system the idea is a command economy. Things are only produced if there is enough want or rather NEED for them. They don't consider social trends, it's all too homogeneous. But in reality nobody cares about such things. If someone wants a TV they aren't going to give a damn about how many people were asked to make that TV or how long it took. They know the workers got paid and most workers will always want to get paid. I haven't seen many labourers actually WANT Communism. Most just want a fair go and more rights.
I don't think a command economy will ever work. Nobody wants to be told that this is the bare minimum they can have. We work to make money to buy things we want. Be it a physical item or a holiday we work for ourselves and we work to please ourselves. We work so we can live. We pay taxes as our contribution to the state. This is the most balanced system. It's not perfect, but if we jump to any extremes expect some kind of crisis. I mean every time a nation has attempted to radically overhaul the current capitalist system they have ultimately met poverty and even more corruption.
It's not just not possible to find any workable alternatives so I don't know why people even bother trying. It's all meaningless. The revolution is over. It failed. Get over it people. Shit. I mean yes, I want reforms. Reforms and more reforms - as many as it takes to help workers and to keep the capitalist system stable and as fair as possible. I call this a realistic economic model.