As an independent developer or author, every bit of time or money put into superficial things like a book cover is time or money that isn’t used for the content itself.
That there's a fixed budget of time/money - exactly X hours/dollars will be allocated to the book, one way or another.
That the improvement to the quality of the content as time/money is invested is something approaching linear - that if exactly X hours/dollars are going to be allocated to the book, the content will be appreciably better if, say, 0.9X is allocated to the content vs, say, 0.8X.
That time and money is fungible: that all time/money that could be invested in the content could be invested in superficial things, and the other way around.
As independent developer.
Yes, time is fixed. There is only 24 hours in a day.