Amazon are selling a 10 pack of Aone DVD+R DL disks for £4.53 delivered.
These are good for xbox 360 backups and Amazons description even mentions using them for Xbox backups. They are full face printable.
DVD+R DUAL LAYER Full face inkjet printable surface
8.5GB / 240 Min capacity
compatible with DVD Players, Duplication Systems, XBOX 360 consoles
Brand new sealed in retail package