Because the remains were stuffed into the magic bag right after killing the dragon, they preserved their freshness. Those would very likely sell for a large amount of money.
Also, because the remains held a gigantic magic crystal within, the kingdom would purchase it for 400 platinum coins.
As a result, combining it with the monster remains of the one week of hunting afterwards, the total amount per person was 450 platinum coins and 50 gold coins when split between Armstrong-doushi, Burkhart-san, and myself.
In any event, all of the dragon’s raw materials were expensive.