Frontier: Checked baggage $25 web / $30 at airport. Carry on $30 web / $35 at airport. Gate check $50. Basically you're punished for gate checking, rather than committing in advance to either a carry on or checking before security. And it means no matter what you're paying for anything other than a personal item.
The Frontier gate check fee seems to only apply to items that are not eligible to be carry-on. It would not make sense to charge to gate check a carry-on item (as you are often required to do when the overhead bins are full).
