I'm agonizing about buying back-to-backs when we pick up ours in IST next month. We'd be finishing the pending trip around November, so IMO the chances are very good that by then the price may well have gone up, so locking in might be the wisest thing to do.

It seems a fairly straightforward "time value of money" calculation - loss of interest earnings on cash used for tickets plus interest expense (if any) on amount charged to credit cards but not paid off right away, plus currency conversion charges if any (not a problem for us if billed in USD, 3 or 4% if converted), less potential increase in ticket price, plus/less changes in taxes, fuel surcharges, etc. that might occur during the "float" period.... and so on.

As with most such purchases the general rule is that it's never going to be as cheap as it is today, and most investors will tell you that trying to time the market is a fool's game. But it's a big chunk of change and we too are wondering about timing.