High fixed costs can be a “killer” if you don’t have them under control. It sounds obvious, but this point hit home from my first-hand experience. I would emphasize the marketing cost since we self-publishers need to do promotion ourselves.
For example, printing a postcard of decent quality costs 10 pence. To distribute it to an apartment building with 100 units, you have to shell out 10 pounds without knowing the impact on book sale. For argument sake, if the profit of each book is 1 pound (hypothetically), to break even, 10 postcards need to generate the sale of one book! Based on this simple calculation, a more focused marketing target would make more sense unless you can reduce the printing cost, provide a wider product range and/or increase the profit margin.
In short, eyes on fixed costs before “they eat you alive”.