It might not affect the price at all as the price is mostly controlled by an accumulation/distribution cycle of whales.
Would you be willing to place a 0.5 BTC bet?
If the price goes up during the 24 hour period after Amazon announces they are accepting bitcoin, you give me 0.5 BTC.
If the price goes down or remains exactly unchanged during the 24 hour period after Amazon announces they are accepting bitcoin, I give you 0.5 BTC.
If Amazon never accepts bitcoin, the bet is void.