You don't know that's the problem, If you really want it backorder it from all of them, so these here:
pool.com
snapnames.com
Godaddy.com
clubdrop.com
I believe the only one that charges no matter what is godaddy, the others only charge when you or if you win I believe. but the best that I've used is snapnames, the drops I tend to win are almost always with snapnames.
Remember each one of these represents different registrars for example:
ClubDrop.com:
is Enom so if the expired/drop domain was originally with enom or a enom reseller you run a better chance catching it through clubdrop.com
Snapnames.com:
partner with Network Solutions, moniker, Dotster, and a few others, my #1 in terms of success with drops.
Godaddy.com:
partner with themselves... they suck anyway so whatever. Stay away from them at all costs is my recommendation.
Pool.com:
I don't believe they partner directly with anyone anymore but they do still catch some drops well, not what they used to be.
So in a majority of the cases you will win the backorder with the partner site.... most of the time. It's a weird process.
If more than one person backorder the domain with the same backorder service the service if it wins the backorder will then put it up for auction between the people who placed the backorder.
the backorders at snapnames have cost me a minimum of $60 US, if it didn't go to auction after the backorder.
Hope this helped you a little, for more info on pricing visit each of the backorder sites.
oh I almost forgot I believe tucows.com also does backorders but I've never dealt with them so I don't have any experience to share with them
Good Luck!