About the empty corridors, well in DanMachi it's cause The Dungeon is freakin massive. In Michio's case, his paranoia means he actively aims for the least crowded spots, and with Roxane's nose, it's easy to find 'em.
And yeah, in case of money, he is never broke broke. Afterall they are speedin' through the labirynths compared to the locals (remember that nice guy that spent years on the 7th floor or so?), and that's still slower than they could climb it. But aside from not wanting to take on too much physicall risk, MC also don't want to catch too much atention to himself, so that's not changin'. And they are still livin' quite luxuriously, and he still saves some for the next slave, and proably have a little finanncial cushion in case of emergency, he is just picky 'bout things he decides to spend on (in a different way than Roxane), so like he said, in worst case he will just have to buy a fitting equipement as it appears.