Dabs are currently selling these dual layer +R discs for £2.91 for a pack of 5. They are also selling a pack of 10 for £19.81 GO FIGURE!!
postage is £6 for a single item or you can order more packs (I have ordered 4) and the postage is still £6 so 20 discs plus postage is £17.65.
These discs are 100% proven to work with the XBOX 360