They might just want to get rid of them. That is the only reason why I see this being cheaper than the regular version. See if you can do an instore pick up since many people had problems with the LE since MS cheaped out on the design of the tin case I guess. Discs would come loose from the holdings and would be scratched (I myself experienced this and many other people have to). Open it up at the CS desk when you pick it up just to make sure nothing is scratched and if it is ask if you can get a new one. If they won't let you exchange or getting it through the mail is the only option then you can always send it back to MS for them to send you a fresh new copy and 1 month xbox live card for the inconvience. The turn around time is usually 2 weeks I think. I am not sure is MS is still doing this for free or not though. Good luck OP
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Also, check Gamestops/EB Games. They have Halo 3 used for $39.99 and with the 25% coupon and if you have the edge card you pay $26.00 + tax which isn't bad at all