The iPhone 3G 8GB Refurb is now only $49. It is the only iPhone 3G currently available as a refurbished phone, as AT&T has sold out of the 16GB model.
AT&T notes that supplies are limited, and they expect to sell out soon. You can buy one at the link below.
Update: The 16GB iPhone 3G is once again available, also from the link above, for $99 shipped.