The value of money seem to change quite a bit. In Chapter 26, at page 11 both translations made it seems that just 1 platinum coin would have been enough to live comfortably.
But in this episode she says "It's 20 platinum coins that are each equivalent to 100 gold coins", which would make the exchange rate 1 platinum to 5 gold (the other translation up it to 1 platinum to 10 gold, but it isen't too different for the thought experiment).
With those rate, a tarrasque extermination would make 60-200 gold (12-40/6-20 platinum). Even divided between a party of five or six, at the lowest sell price, it would give at least a platinum coin.
But maybe adventurers want more than simply a quiet life. Let's see for someone else, someone more common, like a restaurant owner for example? Chapter 5, page 32. Loris says his restaurant has 15 gold of debt. So they are 3/1,5 platinum in debt. A sum that, by the words of a bronze level adventurer would allow him to leave for the rest of his life, is less than what a restaurant has accrued as debt. And he is expected to pay 3 gold coins (0,6/0,3 platinum) in a week?
The simple fact that entering the dungeon as a non-adventurer has the possibility of earning that much, in such a short amount of time (the MC never thought it would have been impossible, he just thought would have been suicidal going alone), would mean that with not a lot of time working that way people would be able to earn enough "to live comfortably".
All this to mean what? Fantasy economy does almost never make sense, and avoid stupid phrases as "it would allow me to play around for the rest of my life".