Software Engineers (a large portion of HN community) inherently have a hard time with this. We're always taught to consider edge cases. Oftentimes handling the edge cases can be more work than the rest of the project.

But when we handle them, we give ourselves a pat on the back, without thinking can I just ignore the edge case and shrug if someone runs into it?

In the case of the OP, the edge case is the occasional user who might want to project their exact costs.