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6th Ave via eBay has the Canon EF 24-105mm f/4L IS USM for $639.99. They are not an authorized dealer, but this is very close to the lowest price we’ve seen for this lens, which was $630 during Winter 2013. This is a split-kit lens (taken from a 5D Mark III or 6D kit).
If you prefer authorized dealers, BuyDig still has the lens for $689. It is also a split kit, but BuyDig is an authorized dealer.
Camera Canada has it for $599 CDN as part of their Facebook Specials.