As long as you keep passing, there is no extra cost.
Actually, there's no FCC requirement that you must continue passing to get the next test, just like there's no explicit requirement that you must pass the tests in sequence. It's just that unless/until you pass the earlier tests, you can't earn the higher license--you just get a CSCE.
Some VECs may impose other rules, but ARRL VEC does not. Their VE manual explicitly states that:
- "Each time a candidate pays one test fee, he/she is entitled to take tests for Elements 2, 3 and 4 as needed, up to all three under
the single test fee." Note that there is nothing that says "as long as he passes the previous ones." (page 42)
- "An applicant may take a series of exams in any order to qualify for a license." (page 49)
Now they do recommend
in a number of places in that manual that the nominal ascending sequence be followed, and that applicants pass a test element before moving on to the next. But those are just recommendations, and they do state that their office "will honor the VE Team's decision" in cases that don't follow the recommendations.