If I remember correctly (and its been a while since I've read Das Kapital), Marx tried to explain why a good costs what it costs, where its value really comes f

If I remember correctly (and its been a while since I've read Das Kapital), Marx tried to explain why a good costs what it costs, where its value really comes from. So he was dissecting the different parts that went into its creation only to conclude that they really weren't different parts, but that it all came down to labour in the end - basically what you said. His theory however did not explain why the value of a good fluctuates over time, like the price of bread in times of bad harvests, and I have some fuzzy memories of weird corner-case explanations to maneuver around this issue. Today we know that no matter how much human labour went into the creation of something, in the end the laws of supply and demand determine its price. I could be wrong though and thats not what Marx goal was, after all.