It's your own one-way ticket price. Luckily, the one time ended up with an IDB on Frontier the ticket wasn't crazy cheap, and they even had a flight with open seats later in the day. So I ended up with a $600-something check for taking a flight 6 hours later.

If you paid in miles and end up with an IDB, you're due 4x the lowest paid ticket in the same class. That could potentially be a pittance on Frontier, so make sure to check-in early on award tickets in the off chance it happens.