It really depends on you and what you're comfortable with paying, especially since you can't get a full refund with most airlines. By that, I mean they still want to charge the change fee before refunding the difference.
In any event.... we are going for three weeks starting on June 14. We booked our flights in January. I believe we paid about $350/ticket. We then had a huge schedule change. As a result, we re-booked with no change fee. As a result, I think our ticket value went up to about $390/ticket. I keep checking our flight to see what the price is now. At last check (a few days ago), our same flight is now going for something like $890/ticket.