I mean, would you buy a freshly mined gold ore with the price of a 24 Karat minted gold of the same weight? Probably not right. It is one thing if he quality check it first and sell it to auction, but that is his choice to sell it at base price for raw material.
That...is a VERY bad analogy.
Gold can be found relatively easy.
The MC found an one of a kind raw material, something that as the guild master put it, can be considered a national treasure. Based solely on its rarity the MC should have been set for life.
In this case we are also speaking of raw material at a size that can produce weapons litteraly only seen in legend.
So yeah he got scammed big time,and it wasn't really his choice, the guild master outright coerced him, by falsely giving him only two options, sell it to the guild for pocket change or for commonly found monster stones.
Completely forgoing other options, such as selling it in auction for example.
Because people would fought and spend millions of coins to have such a unique raw material in their hands.