This has happened to me about a dozen times with DL over the past 9 months and can be quite infuriating. Don't remember it ever happening with NW, KL, or UA (if it did then not often enough or a big enough diff to stick in my mind.) I'd think integrity would say that if they quote you a fare then they should honor that fare for the time it takes to complete the booking process. 15 minutes would be a good timeout I'd think.