What I actually charge for

Design is not the expensive part. Decisions are.

A typical build breaks down roughly like this: a quarter of the time is understanding the business well enough to make the interface obvious, half is building it, and the last quarter is the unglamorous work of making it hold up — states, errors, edge cases, the stuff that decides whether the thing survives contact with real users.

The deposit exists because the first quarter happens before there is anything to look at.