“I would've had to pay a hundred-dollar fine if it had been late.”
You rented a car and returned it at the last minute. The car rental company charges $100 for returning cars late. You say this to your friend about how close you were to being late.
I would've had to pay a hundred-dollar fine if it had been late.
You use this phrase when you're imagining how things would be different if a certain event had happened differently:
I would have gotten home already if I'd left at 7o'clock.
"Would've" is a shortened version of "would have". "Would've" is common in casual spoken English.
When you're talking about an item and want to say how much it costs or is worth, you can say that it is "a ___-dollar ___".
This is a similar expression to "an 8-hour drive". Like that example, you should be careful to use the singular:
He says he bought a 90-dollar tie.
Not "He says he bought a 90-dollars tie."
A "fine" is money that you have to pay because you did something wrong. For example, banks make you "pay a fine" if you try to take out more money than you actually have in your account.