Tiger Airways says at the FAQ:
"Our airfares are quoted in tiers tagged to market demand. Hence, we recommend that you book as early as possible. Generally speaking, the earlier you book the cheaper the fare will be. We always start by selling the lowest fare. Once that is sold out, we sell the next lowest available fare etc. All fares listed are on a one way basis."
But it seems like not the case, at least for mine. After I booked 2 tickets to Hanoi with Tiger Airways, I noticed today that Tiger Airways is offering even lower fares now, with the starting fare of just SGD $9.98. So, everybody that buy the ticket later than me can fly cheaper than me. It makes me furious.
Actually before I bought the tickets, the Tiger Airways air fare to Hanoi actually stands at SGD $69.98 for the dates I want. Then the promotion of from SGD $19.98 to all their destinations came, and now they made it from SD $9.98 to all their destinations. And all these are for the same period of travel. It seems like Tiger Airways really get the demand estimate wrong.