It say 50% off, not $5 off.
I think it has unlocks for Halo 4. Those people might be interested in it.
Maybe it was -$5.00
The price was recently reduced to $10 from $15.
EDIT: I forgot to mention in an earlier post, X1 edition also includes DLC that I think you have to pay for on other platforms.
Edit: looks like they changed it, it says now 9.99 on the dash instead of 50% off.