I feel very comfortable booking in the 400s. We were never able to take advantage of any lower fares and we're ok with that. I have booked as high as $540 (acknowledging that "high" is relative) but that trip fares came down and at the time American gave a refund in the form of a voucher. Don't believe we ever used it because we started flying JetBlue. We have been able in the past to use vouchers but only a couple of times for another trip.

Personally wouldn't book at $600 or higher.