Its all supply and demand. I always tell my customers to book as soon as they know they are traveling. 6-7 months in advance is what I tell them and that is what I follow as well. The different airlines have classes, once that class is all booked they go to the next class etc. The person sitting next to you will most probably have paid a different price. The prices also change from hour to hour. I once book a flight for someone where the usual price is 1500.00 One morning while checking for that person, that flight went down to 750.00. within 2 hours it had gone back up again.
You can actually go crazy waiting for prices to down. So I say to you book ASAP and don't keep checking. And don't necessarily book the cheapest fare, Then you find out when you get there you have to pay for luggage food etc.