If you'd like copies or the source code, just let me know.
JavaScript, for the so inclined:
Javascript Ad Cost Calculator
And for those who prefer to run "script-free," a PHP version:
Lean-and-Mean PHP Ad Cost Calculator
I did SOME input checking, not a ton. It'll catch most things. I figure, you know what you're using it for, so I don't need to check everything.
Let me know what you think!