There are two interesting points here:
The first is monopolies and, in connection, state fixed prices (or price ranges). While monopolies are generally frowned upon in capitalistic economies, exactly because they'd allow the provider to set whatever prices they'd want, it should be noted that for a number of products, monopolies exist naturally.
Think of products and services such as electricity, heat or public transportation. Many countries or regions only have one of these, essentially giving the providers a monopoly. If you feel electricity prices are too high, you do not have the option of switching to a different provider, so you either pay or go without power. Since infrastructure essentially prevents competition in these areas (you can't really run two electric cables to every house), the only thing that can be done is the state fixing the prices for these products... which is a socialist feature.
The point I've been trying to make is, most economies in the world currently are basically midway between a controlled and a free market. Some things are state controlled, some are left to roam free. How much the imaginary slider moves towards one direction or the other depends on many things, most notably the general economic situation of the country; ie. richer countries tend to have a more free economy, because the people can generally afford to buy overpriced things.
The other thing people seem to be misunderstanding is the idea behind communism. See, the basic idea there was that eventually society would change in a way that everyone would basically just share everything necessary to live; there would be no currency in this situation! Of course, getting to this state would be fairly difficult: the idea was that there'd be a temporary phase, during which there would be a government with fixed leaders. They'd work to turn society into this ideal and once it's been accomplished, they'd step down, since there'd no longer be a need for a state or leaders.
So the basic timeline Marx thought up was...
1) A communist revolution
2) A temporary phase, when currency and leadership still exists, setting up...
3) True communism, a utopia where everyone is equal and resources are shared, rather than sold and bought
Of course, in reality, the third phase was never reached. One could argue this was because the idea of communism is, in itself, flawed; or one could argue it was of more practical reasons, such as incompetent leadership, poor handling of problems and simply the fact the leaders weren't actually trying to reach true communism.
In the above, Marxist sense, "communism" never actually existed during history and still does not. Correctly, only "socialist" states existed (and exist to this day). But just to get back to the topic, pre-economy Dwarf Fortress is actually very much like what Marx envisioned as a true communist society.