Be very wary of these ticket agenices. They're not the official suppliers.
Most are ticket touts, posing as genuine ticket agencies. They charge much more than the ticket face value and hope that they will be able to get hold of tickets nearer the time. They don't actually have the tickets they're selling. In the case that they can't get hold of the tickets last minute, some, (if you're lucky) will give you a refund, but as in most cases, you'll probably get nothing.
There was a documentary about them on TV a few weeks ago (The One show I think?).
I got caught out that way with the Onlineticketshop.com.
Don't panic, these are not the genuine ticket prices! They are simply rip-off merchants trying to cash in! Think about it - if MJ hasn't even confirmed to O2 if he's going to perform, how can they be offering tickets?
Make sure you but tickets from 02 or genuine agencies like Ticketmaster.