The SAT II Math isn't the only test. The normal SAT math component also allows calculators and is generally administered on different dates. The SAT is offered on 21 dates in 2019, so that's a total of 27 tests. Even at a full ~$100 per calculator, charging an extra $1 per test would still cover the cost in a little more than 3 years.

AP Exams: Unlike the SAT which has regional test locations, AP exams are generally administered in the school you attend, and the school should provide a calculator. For that matter, the whole issue could be solved if schools issued them as standard equipment that same way they do textbooks.

Finally, the whole point of these tests is to establish the level of knowledge a student has. That hasn't really been done if the test takers aren't on the same level playing field. All else being equal, the same score for a test taker with a calculator compared to one without would mean very different things. It's supposed to be a standardized test. All students should have the same equipment.

Adding any more logistics into this process would increase costs and provide no headline-worthy benefit.

* The teachers, not the students, use scantrons

Either way, the headline-worthy benefit, if I take your meaning with that phrase correctly, is that this is a standardized test. You don't get to "standardized" by not providing equal equipment to test takers. Regardless of whether it's "easy" to do, it is a necessary condition for actually being a standardized test.