I am still unsure about the SAT Subject Test costs. I know that I have to pay a basic registration fee of $26. But if I want to take multiple exams on the same test date, do I have to pay any additional fees? (I'm taking US History and Math Level II if that helps.)
The $26 basic registration fee doesn't cover any of your Subject Tests on its own. You have to pay $26 plus an additional $18 for each regular (non-listening) Subject Test and an additional $26 for each Subject Test with a listening component.
In your case, you're taking two regular subject tests, so you'll pay $26 for basic registration, $18 for the US History test, and $18 for the Math II test. That means a total of $62. For more information on the cost of Subject Tests (and what you would have to pay for special conditions like changes to your registration), check out these charts on the College Board site.