Mertan's page

They buy food to nearby farming settlements so they don't produce their own as of now.
I don't think the players want to go full modern on the settlement, i think they are more worried about making it up to "decent but medieval" as in available water, roof and fire for everyone etc. Plus walls, yes, walls.
They are also training/recruiting promising kobolds and some allied Hobgoblins to have a group of "adventurers" that can handle missions below the scope of the party while benefiting from the party's fame and hence charge decently for it.

It duplicated the workforce (more kobolds) and the income has grown about 50%.
They are now paying tribute to a dragon nearby so it won't attack and aid in defense.
It has a Kobold town on the side (many many kobolds).

Did you tell them that this was actually in an outer plane?

So this is where you've been getting the ideas huh? hehehe
He is right, using the money for fluffy RP things or consumables etc, will make this whole, the party has a mine thing go a lot more smoothly than spending the income in weaponry or magical items. The mine was put there as a roleplaying incentive not as a dress up your fighter machine.
Some very nice suggestions btw. I say they need a flying carpet.