Amazon didn't jack up the price for the pile drivers.
That 8000 yen price is from one of the 'marketplace' sellers. It often happens on amazon japan I have noticed, a preorder goes up and is soon sold out. Then on release date, you suddenly find that another place selling through amazon happens to have a lot of stock on hand of an 'amazon exclusive' but the price is almost double (or more) than the original.
I suspect the exact same thing has happend to amazon excl. Soundwave as well as a pre-order can't be placed on that.
It happend to US Excl. Unicron and is probably what happend to the G2 Combaticons as well.
Scalpers