I'm amaze on how airfare can change so drastically. I was reviewing a flight to Aruba from NY which the other day was like $450 one way. Today it was $238 but only 1 seat available. I priced other dates and then went back to the flight that offered only 1 seat at $238 and now there were 5 seats available at $238.00. Amazing! The round trip was around $900 the other day and now like $700. Tomorrow or 1 hour from now it will be a different price. This is on jet blue.
I saw a show on TV once where they were explaining that they use very sophisticated computer program that changes the prices depending on how many seats are booked on that flight for that day. Sometimes if you happen to be on at the right time you can get a super cheap fare but it doesn't last long. Twice I saw mint for $445 one way when they were selling them for $1345 one way but the offer lasted 1 hour in both instances. One time I remember some passengers I think on United got round trip airfare for like $35 which was a computer glitch but they had to honor it.
My motto is that if you see a great bargain, book it right away if you are sure of your travel dates because it won't last. Except I have to confirm the hotel first and then I book the airfare but it doesn't hurt to look and see what the airfare is.