The economy of a game is ridiculous important, but the basic concepts aren’t difficult. You’re balancing how much things cost with how much people think they’re worth. Now it can get a little weird if you’re hiding the value behind leveling up to earn new things. It’s not as simple as 1+1 = a new Fish, that’s part of the game. Value perception happens sooner or later with your users.
Breaking this down in sections since each is important in their own right and can be complex. They are together a thing so it’s important they know each other. In other words they balance together.
If I come up with a sick store concept, but my experience curve doesn’t match it doesn’t matter. If I don’t give players new things (content) at a rate that matches their play and spend they’ll leave to.
While we all (or most) want to make fun games they don’t create themselves for free. These concepts are always changing so it’s important to always be trying new things. Nothing is “right” in a live product.