While you are technically correct that supply and demand are what determine the price of any given good, that's not the point.
While I'm sure you can come up with any number of bizarre scenarios in which historical importance doesn't actually drive up the demand, the fact is that in this particular case, the historical importance of such an object is likely to significantly contribute to its demand (hence the high price paid). The high demand for it was clearly not due to its productive capacity -- thus a reasonable conclusion would be it was due to its historic importance. Its historical importance is therefore instrumental in the determination of the monetary value.
All I was pointing out is that your statement, "An important place in history has little to do with monetary value." is simply not true. Does historical importance ALWAYS determine monetary value? Obviously not (your mud example provides excellent support for this point). However, often the historical importance of an object does indeed significantly impact the monetary value by driving up demand for it, as is most likely the case here.
P.S. And if you really want to get nit picky, "Supply and demand determine price. Nothing else." isn't technically true anyways. There are plenty of cases in which supply and demand don't determine price. Minimum wage?