I did my best to add a bit of everythings since the shops is core gameplay and i wanted to view it filled with goods.
For the difficulty I probably mess up somethings since i planned for something more or less linnear so that shouldn't have happenned, knowing me i probably invert + and * again while doing the formula my bad XD (and i didn't really play-test for the difficulty curve), i'm going to patch it for next update along with the tutorial, thanks for your feedback