.com domain price is only $6 to the registrar.
$6.39 is very doable. ( bet they don't stay that price or are high on the addons)
all depends how greedy the registrar wants to be, the amount of features they offer with the domain vs charge for extras, etc...
$8ish is a FAIR price if it includes self service features like forwarding.
(uses some their pc resources)
sometimes they will "sale" lower than cost, and hope to make it up on other services they sale.
I generally pay $7
although I can get a CONSTANT $6.49 on a reseller account, which is less pretty and more manual.