Very often
Recently I was buying a Memory Stick for a friend, and it was around $19.99 for 1Gb. He didn't make his mind then, so the next day, he says "let's buy it"... I go in, and it costs $30+!!!! 😱
Fortunately we searched somemore and found a 2Gb for ~$22, and that's what he got. But yes, Amazon does change the prices very often... even several times in the same day!