It's funny, because I saw a similar thread in the SCJA forum.
Here's what I think: Prometric lists a price that they THINK it is, or they are told it is at some point. Then, Sun decided on the 'official' price, and the two are out of sync.
However, when you do buy through Prometric, you get redirected to Sun's site. I think you can only buy the voucher directly from Sun, as there are no official resellers. From there, you are charged the actual, official price, from Sun.
If anyone's experience differs from mine, please speak up. But from what I understand, all voucher sales go through Sun's site in the end.