2568 books at under $5 a pop plus flat rate $6.50 delivery from Booktopia
Booktopia are running their 9th birthday sale. Not sure how long it runs for but they have 2568 books at under $5 in the listing. Credit to them, they have flat rate shipping per order of $6.95. As with any sale of this nature, in the $5 bucket theres a whole bunch of pulp, but there's the odd good thing and you can always up the price range and correspondingly the quality.
Quite a few kids books at $2 or so there.