The MC is building a money base because it gets really expensive to get the best gear when the RPG side of the world comes, plus he has that mountain that would give him tons of money as well because it's a soon to be dungeon which he can put a toll on it.
Time to open up the inventory to store all that water. I guess it would be cubic feet or meters. Get that new skill, " Master of Water" can form water into solid cubes. That would be a cool skill. Since it seems he gets a new skill every dungeon Reset.
In the novel, the values were much different. They spent 22458 yen on the items and sold it for equivalent 1019000 yen. Which is 45 times in value, maybe they changed it for the manga. The chapter where I've found this is on 28.