I've heard of some companies sending out replacement discs if it's still under warranty. I don't know how old this game is, but if it's sorta new, it might be worth checking out the companies website for replacement info.
From what I've read, if they take it, you send them the disc, the CD-KEY #, proof of purchase, and a check for a small fee (usually $5-$10).
If you've got a CD-Burner, or know someone who does, you might be able to make a copy. The trick would be to set up the software to read a full image of the disk (CloneCD would be my recommendation). Make sure that it reads the CD at the slowest speed possible. The higher the speed, the more forces are at work on the CD. With a crack that big, if it gets up to full speed in a modern CD-ROM drive it could very well tear itself apart (more info on that here).
If it survives that, you've lucked out; burn the image and toss the original.
Firstly, the only cavassing of users you should be doing is with a heavy tarpaulin, a stack of bricks and a deep stretch of water" -- BOFH speaks the truth about surveys
Off topic I know, but I know that sites like xbox-scene.com were flooded when the system came out. Recently made systems are being made more stable it seems, but it's still a major issue to anyone that bought one around launch.