It would be useful to point out somewhere that the Small Mana Gel spent to upgrade the Inventory slots increases by an exponential of 2 every upgrade (if i'm not wrong). While the 'this amount seems insufficient' prompt is enough to say it's not enough, it doesn't tell the player how much they are missing.
Right now having to spend 64 Mana Gels to get another gear slot feels just a bit too much and I only got 5 gear slot expansions. The next one will cost 128 Gels... ( '-_-) seems like I won't be leaving my Plush Suits anytime soon.
Maybe the upgrade cost could be additional instead of exponential, like starting at 6, getting increased by 6 every upgrade and you could limit the expansions by requiring a second material to level it up, like the Blue Radiant Stones, that for the moment serve no purpose.
The mines could also have more levels/floors, so you could harvest different materials on each floor. Maybe fight some different enemies, discover new stuff?