well, enough talk of root canals. My teeth are beginning to hurt. lol
Really, I would bite the bullet and fix it. It's a great lens. It shouldn't fail again. Whatever did this was most likely a manufacturer defect that went unnoticed. It probably caused something to wear abnormally which led to the failure. Now fixing it is expensive, but those related parts should all be repaired and it should be as good as, or better than new.
$640 isn't bad for this $3000 lens. If it bugs you, you can buy a 7 yr mack warranty from Adorama. At the very least, you could fix it and sell it - just making sure you disclose the repair of course. Repaired, I wouldn't hesitate to spend $1500 on it and I would have to dig for the money. Others with the case more readily available may pay even more. But if you really really don't want to mess with it, send me the repair estimate and I'll buy it as-is for $500 and pay for the repair.
And I wouldn't let this stop me from buying Sigma lenses. If you are happy with the lens, often even buying it a second time can cost less than the Canon counterpart.
I am serious....and don't call me Shirley.
Canon 7D and a bunch of other stuff